dedicated - eprints · iii acknowledgements all praise to almighty allah for his mercy upon us and...

ii

DEDICATED

TO MY BELOVED PARENTS,

GRANDPARENTS, WIFE,

SIBLINGS AND MY EXTENDED

FAMILY

iii

ACKNOWLEDGEMENTS

All praise to Almighty Allah for his mercy upon us and guiding us towards the right path. My

immense appreciation goes to my mother; Mrs. M.I. Gbadamosi for her support. My siblings,

Abimbola, Afolake and Abisola have also been wonderful in their support guiding me since cradle.

My wife; Mrs. Bilikis Gbadamosi level of support cannot also be quantified. I also extend my

gratitude to my friends, in-laws, immediate and extended families for their support. May Allah also

blessed the soul of my Daddy, Mr. S.A. Gbadamosi, my granny; Mama Halimatu Busari and all the

dead ones who have also contributed positively in one way or the other to my growing up before their

demise.

My appreciation also goes to my thesis advisor, Dr. Zuhair M. Gasem for taking out of his tight

schedule time to contribute immensely to my success in the MSc. programme. Likewise my thesis

committee members, Prof. A.F.M. Arif and Dr. S.S. Akhtar contribution is unquantifiable. My

appreciation also goes to Dr. Khaled Alathel and Dr. Jafar Albinmousa who also contributed to the

success of my research work.

Words can’t also quantify the contribution of Nigeria community, African and Asian communities in

KFUPM; and also my friend in all walks of life.

Thank you all. Allah will bless you abundantly.

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TABLE OF CONTENTS

ACKNOWLEDGEMENTS.……………………………………………………………………..iii

TABLE OF CONTENTS.………………………………………………………………………..iv

LIST OF FIGURES………………………………………………………………………………ix

LIST OF TABLES……………………………………………………………………………...xxv

THESIS ABSTRACT (ENGLISH)…………………………………………………………...xxvii

THESIS ABSTRACT (ARABIC)……………………………………………………………..xxix

CHAPTER 1………………………………………………………………………………………1

INTRODUCTION………………………………………………………………………………...1

1.1 Coatings…………………………………………...………………………………….…….1

1.2 Types of Coatings……………………………………………………………………...…..1

1.3 Thermal Barrier Coatings (TBCs)...…………………………………………………….….3

1.4 Methods of TBCs Deposition……………………………………………………………...5

1.5 Hot Corrosion………………………………………………………………………………5

1.5.1 Hot corrosion of 8YSZ in the Presence of P2O5 contaminant…………….…….......7

1.5.2 Hot corrosion of 8YSZ in the Presence of Na2SO4 contaminant…………………...7

1.5.3 Hot corrosion of 8YSZ in the Presence of Na2SO4+V2O5 contaminants…………..8

1.5.4 Hot corrosion of 8YSZ in the presence of V2O5 contaminant….…………………14

1.6 Reaction of Calcium oxide (CaO) and Magnesium oxide (MgO) with V2O5……..……...19

1.7 Motivation and Objective…………………………………………………………………22

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1.7.1 Motivation………………………………………………………………………....22

1.7.2 Objective…………………………………………………………………………..22

CHAPTER 2……………………………………………………………………………………..24

LITERATURE REVIEW………………………………………………………………………..24

2.1 Alternative Stabilizers for zirconia compound…………………………………………...24

2.1.1 Ta2O5-stabilized Zirconia...……………………………………………..…………25

2.1.2 Sc2O3-stabilized Zirconia (SSZ)…………………………………………...………26

2.1.3 MgO-stabilized Zirconia (MSZ)…………………………………………………...28

2.1.4 CeO2-stabilized Zirconia (CSZ)............................................................................... 30

2.1.5 CaO-stabilized Zirconia (CaO-ZrO2)………………………………………………32

2.1.6 In2O3-stabilized Zirconia…………………………………………………..………33

2.1.7 SnO2-stabilized Zirconia…..………………………………………………………34

2.1.8 TiO2-stabilized Zirconia…..……………………………………………………….35

2.1.9 Yb2O3-stabilized Zirconia (Yb2O3-ZrO2)…..……………………………………...35

2.2 Pyrochlores……………………………………...………………………………………..36

2.3 Ceramic Composites………………………..…………………………………………….37

2.4 Overlay Coatings…………………………………………………………………………38

2.5 Surface Sealing………………………………...…………………………………………41

2.6 Fuel Additives Inhibitors………………………..………………………………………..43

CHAPTER 3……………………………………………………………………………………..48

EXPERIMENTAL PROCEDURES……………………..………………………………………48

3.1 Materials……..……………………………………..……………………………………48

3.2 Equipment………...…………………………………..………………………………….49

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3.3 Inhibitive Hot Corrosion Experiment………...……….…………………………………50

3.3.1 Inhibitive Hot Corrosion Experiment (Powder Samples)..……….………………50

3.3.2 Inhibitive Hot Corrosion Experiment (APS 8YSZ-TBCs Coupon Specimen)..….51

3.4 Characterisation Methods………………………………………………………….…….52

CHAPTER 4……………………………………………………………………………………..61

RESULTS.……………………………………………………………………………………….61

4.1 Effect of Isothermal heating in Hot Corrosion Test……………………………………..62

4.2 Vanadium-induced Hot Corrosion of 8YSZ Powder……….……………………………62

4.2.1 Colour Spectrum Analysis…....…………………………………………………..62

4.2.2 XRD Analysis…………………………………………………………………….63

4.2.3 SEM and EDS Analyses……………………………..…………………………..66

4.2.4 X-ray Mapping Analysis….……………………………………………………..67

4.3 MgO-Inhibition of Vanadium-induced Hot Corrosion of 8YSZ powder with V2O5

concentration (10% wt. 8YSZ)……………………..…………………………………...68

4.3.1 Colour Spectrum Analysis…...…………………………………………………...68

4.3.2 XRD Analysis.……………………………………………………………………69

4.3.3 SEM Analyis…...…………………………………………………………………78

4.3.4 EDS Analysis……………………………………………………………………..78

4.3.5 X-ray Mapping Analysis………………………………………………………….84

4.4 MgO-Inhibition of Vanadium-induced Hot Corrosion of 8YSZ powder with lower

V2O5 concentration (5% wt. 8YSZ)…….………………………………………………..85

4.4.1 Colour Spectrum Analaysis……...………………………………………………85

4.4.2 XRD Analysis…..……………………………………………………………….90

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4.4.3 SEM Analysis…...………………………………………………………………91

4.5 CaO-Inhibition of Vanadium-induced Hot Corrosion of 8YSZ powder with V2O5

concentration (10% wt. 8YSZ)…………………………………………………………..92

4.5.1 Colour Spectrum Analysis…………….………………………………………..92

4.5.2 XRD Analysis……………………….………………………………………….97

4.5.3 SEM Analysis………….……………………………………………………….99

4.5.4 EDS Analysis………….………………………………………………………104

4.5.5 X-ray mapping Analysis………….…………………………………………...105

4.6 CaO-Inhibition of Vanadium-induced Hot Corrosion of 8YSZ powder with lower

V2O5 concentration (2% wt. 8YSZ)……………………………………………………106

4.6.1 Colour Spectrum Analysis…………..……………..………………………….106

4.6.2 XRD Analysis…………………………………………………………………111

4.6.3 SEM Analysis……………….…………………………………………...........112

4.7 CaO-MgO mixture-inhibition of Vanadium-inudced Hot Corrosion of 8YSZ

Powder with V2O5 concentration (10% wt. 8YSZ)……………………………………116

4.7.1 Colour Spectrum Analysis………………….…………………………………116

4.7.2 XRD Analysis…………………………………………………………………117

4.7.3 SEM Analysis…………...…………………………………………………….121

4.7.4 EDS Analysis………………….………………………………………………126

4.8 Air Plasma Sprayed (APS) Specimen Inhibitive Hot Corrosion Test…………….…...127

4.8.1 Physical Appearance………………...…………...……………………………127

4.8.2 XRD Analysis…………………………………………………………………127

4.9 Inhibition Efficiency…………………………..………………………………………133

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CHAPTER 5……………………………………………………………………………………141

DISCUSSIONS…………………………………………………………………………………141

5.1 The Effect of Isothermal Heating and Cooling………………………...………………...141

5.2 Vanadium-induced Hot Corrosion of 8YSZ powder………………...…………………..142

5.3 MgO-inhibition of Vanadium-induced Hot Corrosion of 8YSZ powder………...……...144

5.4 CaO-inhibition of Vanadium-induced Hot Corrosion of 8YSZ powder…………...……147

5.5 CaO-MgO-inhibition of Vanadium-induced Hot Corrosion of 8YSZ powder………......149

5.6 APS Specimen……………...……………………………………………………………151

CHAPTER 6……………………………………………………………………………………157

CONCLUSIONS…………………...…………………………………………………………..157

REFERENCES……………………..…………………………………………………………..159

VITAE…………………………….……………………………………………………………165

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LIST OF FIGURES

Figure 1.1: Types of Coatings…………………………………………………………………..…3

Figure 1.2: Thermal Barrier Coatings………………………………………………………….….6

Figure 1.3: ZrO2-Y2O3 phase diagram……..……………………………………………………...8

Figure 1.4: SEM micro-structural micrographs. (a) APS YSZ, (b) EB-PVD…………………….9

Figure 1.5: V2O5-Na2SO4 phase diagram……….………………………………………………12

Figure 1.6: Hot corrosion of 8YSZ in the presence of Na2SO4+V2O5…..………………………13

Figure 1.7: AE signal monitoring of destabilization of ceramic topcoat………………………...13

Figure 1.8: TGA and DSC plot for V2O5……….………..………………………………………16

Figure 1.9: Hot corrosion zones in TBCs………………………………………………………..16

Figure 1.10: Phase evolution versus time plot for vanadium oxide hot corrosion of 7YSZ ceramic

topcoat……………………………………..………………………………………17

Figure 1.11: MgO-V2O5 phase diagram………………….…..…………………………………..20

Figure 1.12: CaO-V2O5 phase diagram…………………..………………………………………21

Figure 2.1: SEM micrograph of YSZ and SSZ after 475hrs isothermal heating at 700oC. (a)

YSZ, (b) SSZ……………………………………………………………………….27

Figure 2.2: XRD analysis of YSZ and SSZ after 160hrs isothermal heating at 900oC…….……28

Figure 2.3: XRD analysis of hot corrosion test of zirconia stabilized with 25%wt. CeO2 and

25%wt. Y2O3………………..…………………………………….…………………32

Figure 2.4: SEM micrographs of hot corrosion of 10CaO-ZrO2 in molten fuel contaminant salt

……………………………………………………………………………………...33

Figure 2.5: Volume fraction of monoclinic zirconia in the coatings after hot corrosion test. (a)

YSZ, (b) YSZ+Al2O3, (c) YSZ/Al2O3…………..……….…………………………38

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Figure 2.6: Rate of destabilization versus time plot of as-sprayed ceramic topcoat…………….40

Figure 2.7: SEM surface morphology of ceramic topcoat before hot corrosion. (a) as-sprayed, (b)

CO2 laser glazed, (c) Nd-YAG laser glazed………………………………………...42

Figure: 2.8: Action of Magnesium stearate addition on weight gain for (213T1) steel at 1:1,

dotted line; 2:1, dashed line and 3:1, solid………………………………………….44

Figure 2.9: Effect of contaminants and inhibitor on high temperature corrosion of Nimonic 90 at

850oC…………………….………………………………………………………….46

Figure 2.10: Weight change versus time. (a) Na/V=1.5, (b) Na/V=1.5,Mg/V=3, (c) Na/V=1.5,

Ni/V=2.25 oxidation at 850oC……….……………………………………………47

Figure 2.11: Weight versus number of cycles…………………………………………………...47

Figure 3.1: (a) Phoenix AB-224 weighing device, (b) Mortar, glass tubes and alumina

crucibles………...………….………………………………………………………..54

Figure 3.2: (a) Inhibitive hot corrosion experimental set-up, (b) Hunterlab colour characterization

device…………………….…………………………………………………………58

Figure 3.3: D-8 XRD Machine…………………………………………………………………..59

Figure 3.4: (a) TESCAN LYRA Machine, (b) JOEL JSM 6460LV Machine…………………..60

Figure 4.1: (i) Monoclinic zirconia volume fraction bar chart on effect of isothermal heating on

2hrs-900oC isothermal heated 8YSZ powder, (ii) SEM micrographs of 2hrs-900

oC

isothermal heated 8YSZ powder. (a) Furnace cooled sample, (b) Laboratory air

cooled sample……..…………………………………………………………………65

Figure 4.2: (i) Colour appearance of vanadium-induced 8YSZ powder samples with varying

concentration of V2O5 at 900oC isothermal heating. (a) Isothermal heated 8YSZ at

100hrs, (b) 2hrs isothermal heated 8YSZ+10wt.%V2O5 powder sample, (c) 100hrs

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isothermal heated 8YSZ+10wt.%V2O5 powder sample, (d) 100hrs isothermal

heated 8YSZ+2wt.%V2O5 powder sample, (e) 100hrs isothermal heated

8YSZ+5wt.%V2O5 powder sample………………………………………………….71

Figure 4.3: XRD analysis spectra (i) 900oC isothermal heated 8YSZ, (ii) 900

oC isothermal

heated 8YSZ powder sample with V2O5 concentration (2% wt. 8YSZ). (a)

As-received 8YSZ, (b) 2hrs isothermal heating, (c) 5hrs isothermal heating, (d)

20hrs isothermal heating, (e) 50hrs isothermal heating, (f) 100hrs isothermal

heating……...………………………………………………………………………72

Figure 4.4: XRD analysis spectra (i) 900oC isothermal heated 8YSZ powder sample with V2O5

concentration (5% wt. 8YSZ), (ii) 900oC isothermal heated 8YSZ powder sample

with V2O5 concentration (10% wt. 8YSZ). (a) As-received 8YSZ, (b) 2hrs

isothermal heating, (c) 5hrs isothermal heating, (d) 20hrs isothermal heating, (e)

50hrs isothermal heating, (f)100hrs isothermal heating……………………………73

Figure 4.5: Amount of tetragonal-zirconia phase transformation against time for vanadium-

induced hot corrosion 8YSZ powder samples with varying V2O5 concentration

at 900oC isothermal heating. (8YSZ) 8YSZ powder, (2% V2O5) 8YSZ+V2O5

(2% wt. 8YSZ) powder sample, (5%V2O5) 8YSZ+V2O5 (5% wt. 8YSZ) powder

sample, (10%V2O5) 8YSZ+V2O5(10% wt. 8YSZ) powder sample………………..74

Figure 4.6: SEM micrographs of 100hrs-900oC isothermal heated as-received 8YSZ powder and

its mixture samples with varying concentration of V2O5. (i) Lower Magnification,

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(ii) Higher Magnification. (a) As-received 8YSZ, (b) Isothermal heated 8YSZ,

(c) 8YSZ+V2O5 (2% wt. 8YSZ), (d) 8YSZ+V2O5 (5% wt. 8YSZ), (e) 8YSZ+V2O5

(10% wt. 8YSZ)…….………………………………………………………………75

Figure 4.7: EDS analysis. (a) Smooth-face spherical shaped powder particles, (b) Rough-face

spherical shaped powder particles……..……………………………………………76

Figure 4.8: (a) Backscatter/Secondary SEM micrograph of rough-face spherical shaped powder

particles, (b) X-ray mapping analysis of rough-face spherical shaped powder

particles (Vanadium-induced hot corrosion sample)……….……………………….77

Figure 4.9: MgO -inhibitive vanadium-induced hot corrosion 8YSZ powder samples with

varying MgO/V2O5 molar concentration (MMgO/MV2O5) colour spectrum at 900oC

isothermal heating. (a) Heated 8YSZ at 100hrs, (b) Vanadium-induced hot corrosion

8YSZ at 2hrs, (c) Vanadium-induced hot corrosion 8YSZ at 100hrs, (d) 8YSZ

mixture sample of MMgO/MV2O5=1 at 100hrs, (e) 8YSZ mixture sample of

MMgO/MV2O5=2 at 100hrs, (f) 8YSZ mixture sample of MMgO/MV2O5=3 at 100hrs,

(g) 8YSZ mixture sample of MMgO/MV2O5=5 at 100hrs……...………………………79

Figure 4.10: (i) XRD analysis of MgO-inhibitive vanadium-induced hot corrosion 8YSZ powder

sample with MgO/V2O5 molar concentration (MMgO/MV2O5) equals 1 at

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900oC isothermal heating, (ii) XRD analysis of MgO-inhibitive vanadium-

induced hot corrosion 8YSZ powder sample with MgO/V2O5 molar concentration

(MMgO/MV2O5) equals 2 at 900oC isothermal heating. (a) As-received 8YSZ, (b)

2hrs heating, (c) 5hrs heating, (d) 20hrs heating, (e) 50hrs heating, (f) 100hrs

heating……………...………………………………………………………………80


sample with MgO/V2O5 molar concentration (MMgO/MV2O5) equals 3 at

900oC isothermal heating, (ii) XRD analysis of MgO-inhibitive vanadium-

induced hot corrosion 8YSZ powder sample with MgO/V2O5 molar concentration

(MMgO/MV2O5) equals 5 at 900oC isothermal heating. (a) As-received 8YSZ, (b)

2hrs heating, (c) 5hrs heating, (d) 20hrs heating, (e) 50hrs heating, (f) 100hrs

heating………………...……………………………………………………………81

Figure 4.12: Amount of tetragonal zirconia phase transformation against time for MgO-inhibitive

vanadium-induced hot corrosion 8YSZ powder samples at 900oC isothermal

heating. (8YSZ) Isothermal heated 8YSZ powder sample, (5MgO(10% V2O5))

8YSZ sample with MMgO/MV2O5=5, (3MgO(10% V2O5)) 8YSZ sample with

MMgO/MV2O5=3 , (2MgO(10% V2O5)) 8YSZ sample with MMgO/MV2O5=2,

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(1MgO(10% V2O5)) 8YSZ sample with MMgO/MV2O5=1, (10% V2O5)

8YSZ+V2O5 (10% wt. 8YSZ)……...………………………………………………82

Figure 4.13: SEM micrographs of MgO-inhibitive vanadium-induced hot corrosion 8YSZ

powder samples with varying MgO/V2O5 molar concentration ratio at 100hrs-

900oC isothermal heating. (a) Heated 8YSZ, (b) Vanadium-induced hot

corrosion 8YSZ sample (V2O5=10% wt. 8YSZ), (c) Sample with MMgO/MV2O5 =1,

(d) Sample with MMgO/MV2O5=2, (e) Sample with MMgO/MV2O5=3,

(f) Sample with MMgO/MV2O5=5………………..…………………………………..83

Figure 4.14: EDS Analysis of MgO-inhibitive vanadium-induced hot corrosion samples.

(a) Rough-face spherical shaped particles, (b) Lump shaped powder

particles…………………………………………………………………………….86

Figure 4.15: EDS analysis of Ridge-face spherical shaped powder particles in MgO-inhibitive

vanadium-induced hot corrosion samples…………………………………………87

Figure 4.16: (a) X-ray mapping elemental analysis of MgO-inhibitive vanadium-induced hot

corrosion sample (Smooth-face spherical, rough-face spherical and lump shaped

particles), (b) X-ray mapping elemental analysis of MgO-inhibitive vanadium-

induced hot corrosion sample (Ridge-face spherical shaped and lump shaped

xv

particles)……………………………………………………………………………88

Figure 4.17: Physical colour appearance of powder particles for the MgO-inhibitive hot

corrosion samples at 100hrs-900oC isothermal heating. (a) Heated 8YSZ, (b)

Vanadium-induced hot corrosion 8YSZ, (c) 8YSZ powder sample

(MMgO/MV2O5 =2),(d) 8YSZ powder sample (MMgO/MV2O5 =3), (e) 8YSZ powder

sample (MMgO/MV2O5=5)……………………...…………….……………………..89

Figure 4.18: XRD analysis of MgO-inhibitive vanadium-induced hot corrosion of 8YSZ powder

samples with V2O5 concentration (5% wt. 8YSZ) and varying MgO/V2O5 molar

concentration at 900oC isothermal heating. (i) Sample with MMgO/MV2O5=2,

(ii) Sample with MMgO/MV2O5=3, (iii) Sample with MMgO/MV2O5=5. (a) As-received

8YSZ, (b) 2hrs heating, (c) 5hrs heating, (d) 20hrs heating, (e) 50hrs heating,

(f)100hrs heating…………...………………………………………………………93

Figure 4.19: Amount of tetragonal zirconia phase transformation for MgO-inhibitive vanadium-

induced hot corrosion 8YSZ powder samples at 900oC isothermal heating.

(8YSZ) 8YSZ powder, (5MgO(5% V2O5)) 8YSZ sample with MMgO/MV2O5=5,

(3MgO(5% V2O5)) 8YSZ sample with MMgO/MV2O5=3, (2MgO(5% V2O5))

8YSZ sample with MMgO/MV2O5=2, (8YSZ+5% V2O5) 8YSZ + V2O5

xvi

(5% wt. 8YSZ)……..……………………………………………………………..94

Figure 4.20: SEM micrographs of powder particles at 100hrs-900oC isothermal heating for MgO-

inhibitive hot corrosion samples of 8YSZ+V2O5 (5% wt. 8YSZ)+MgO.

(i) Sample with MMgO/MV2O5=2, (ii) Sample with MMgO/MV2O5=3, (iii) Sample

with MMgO/MV2O5=5…………..……………………………………………………95

Figure 4.21: SEM micrographs comparison of powder particles at 100hrs-900oC isothermal

heating for MgO-inhibitive vanadium-induced hot corrosion 8YSZ powder

samples with low and high V2O5 concentration (i) Samples with MMgO/MV2O5=2,

(ii) Samples with MMgO/MV2O5=3, (iii) Samples with MMgO/MV2O5=5. (a) Samples

of V2O5 concentration (10% wt. 8YSZ), (b) Samples of V2O5 concentration

(5% wt. 8YSZ)…….………………………….……………………………………96

Figure 4.22: CaO-inhibitive vanadium-induced hot corrosion 8YSZ powder samples with

varying CaO/V2O5 molar concentration (MCaO/MV2O5) colour appearance at

900oC isothermal heating. (a) Isothermal heated 8YSZ at 100hrs, (b) Vanadium-

induced hot corrosion sample at 2hrs, (c) Vanadium-induced hot corrosion

sample at 100hrs, (d) Sample with MCaO/MV2O5=1 at 100hrs, (e) Sample with

MCaO/MV2O5=2 at 100hrs, (f) Sample with MCaO/MV2O5=3 at 100hrs, (g)

xvii

Sample with MCaO/MV2O5=5 at 100hrs………...………………………………….100

Figure 4.23: XRD analysis of CaO-inhibitive vanadium-induced hot corrosion 8YSZ powder

samples at 900oC isothermal heating. (i) Sample of MCaO/MV2O5=1,

(ii) Sample of MCaO/MV2O5=2. (a) As-received 8YSZ, (b) 2hrs heating,

(c) 5hrs heating, (d) 20hrs heating, (e) 50hrs heating, (f) 100hrs heating………..101


samples at 900oC isothermal heating. (i) Sample of MCaO/MV2O=3, (ii) Sample

of MCaO/MV2O5=5. (a) As-received 8YSZ, (b) 2hrs heating, (c) 5hrs heating,

(d) 20hrs heating, (e) 50hrs heating, (f) 100hrs heating……………….…………102

Figure 4.25: Amount of tetragonal zirconia phase transformation against time for CaO-inhibitive

vanadium-induced hot corrosion 8YSZ powder samples at 900oC isothermal

heating. (8YSZ) 8YSZ powder, (5CaO(10% V2O5)) Sample of MCaO/MV2O5=5,

(3CaO(10% V2O5)) Sample of MCaO/MV2O5=3, (2CaO(10% V2O5)) Sample

of MCaO/MV2O5=2, (1CaO(10% V2O5)) Sample of MCaO/MV2O5=1, (10% V2O5)

8YSZ+ V2O5 (10% wt. 8YSZ)………………………………………………….103

Figure 4.26: SEM micrographs of powder particles in CaO-inhibitive vanadium-induced hot

corrosion 8YSZ powder samples at 100hrs-900oC isothermal heating.

xviii

(i) Lower magnification, (ii) Higher Magnification. (a) Isothermal heated 8YSZ,

(b) Vanadium-induced hot corrosion 8YSZ , (c) 8YSZ sample with MCaO/MV2O5=1,

(d) 8YSZ sample with MCaO/MV2O5=2, (e) 8YSZ sample with MCaO/MV2O5=3,

(f) 8YSZ sample with MCaO/MV2O5=5……...……………………………………..107

Figure 4.27: EDS analysis of smooth-faced spherical shaped particles at 100hrs-900oC

isothermal heating CaO-inhibitive hot corrosion experiment…….…...……….108

Figure 4.28: EDS analysis of rough-face spherical shaped powder particles at 100hrs-900oC

isothermal heating CaO-inhibitive hot corrosion experiment

(8YSZ samples with MCaO/MV2O5 =1 and 2 respectively)………….……………109

Figure 4.29: (i) X-ray mapping analysis of rough-face spherical shaped powder particles at

100hrs-900oC isothermal heating (CaO-inhibitive hot corrosion experiment),

(ii) X-ray mapping analysis of smooth-face spherical shaped powder particles at

100hrs-900oC isothermal heating (CaO-inhibitive hot corrosion experiment)..….110

Figure 4.30: Physical colour appearance of powder particles for the CaO-inhibitive vanadium-

induced hot corrosion 8YSZ powder samples with V2O5 concentration

(2% wt. 8YSZ) and varying MCaO/MV2O5 at 100hrs-900oC isothermal

heating. (a) Isothermal heated 8YSZ, (b) Vanadium-induced 8YSZ, (c) 8YSZ

xix

powder sample (MCaO/MV2O5 =1), (d) 8YSZ powder sample (MCaO/MV2O5 =2),

(e) 8YSZ powder sample (MCaO/MV2O5=3)…………...…………………………..113


samples with V2O5 concentration (2% wt. 8YSZ) at 900oC isothermal heating.

Sample of MCaO/MV2O5=1, (ii) Sample of MCaO/MV2O5=2, (i) Sample of

MCaO/MV2O5=3. (a) As-received 8YSZ, (b) 2hrs heating, (c) 5hrs heating,

(d) 20hrs heating, (e) 50hrs heating, (f) 100hrs heating…………….……………114


vanadium-induced hot corrosion of 8YSZ powder samples with V2O5

concentration (2% wt. 8YSZ) at 900oC isothermal heating. (8YSZ) Heated 8YSZ

powder, (3CaO(2% V2O5)) Sample with MCaO/MV2O5=3, (2CaO(2% V2O5))

Sample with MCaO/MV2O5=2, (1CaO(2% V2O5)) Sample with MCaO/MV2O5=1,

(8YSZ+2% V2O5) 8YSZ+V2O5 (2% wt. 8YSZ)…………………..…………….115

Figure 4.33: SEM micrographs of powder particles at 100hrs-900oC isothermal heating CaO-

Inhibitive hot corrosion experiments of 8YSZ mixture with V2O5 (2% wt. 8YSZ)

and CaO with varying CaO-V2O5 molar concentration ratio. (a) Isothermal

heated 8YSZ, (b) Vanadium-induced hot corrosion 8YSZ, (c) Sample with

xx

MCaO/MV2O5=1, (d) Sample with MCaO/MV2O5=2, (e) Sample with

MCaO/MV2O5=3……………….…………………………………………………….118


heating for CaO-inhibitive hot corrosion samples with low and high

V2O5 concentrations. (i) Samples with MCaO/MV2O5=1, (ii) Samples with

MCaO/MV2O5=2, (iii) Samples with MCaO/MV2O5=3. (a) V2O5 concentration

(10% wt. 8YSZ), (b) V2O5 concentration (2% wt. 8YSZ)……………...…………119

Figure 4.35: CaO-MgO-inhibitive vanadium-induced hot corrosion of 8YSZ powder samples

with varying CaO /V2O5 and MgO/V2O5 molar concentrations colour

spectrum at 900oC isothermal heating. (a) Isothermal heated 8YSZ (100hrs)

(b) Vanadium-induced 8YSZ (2hrs), (c) Vanadium-induced 8YSZ (100hrs),

(d) 8YSZ sample (MCaO/MV2O5 : MMgO/MV2O5 =1:1 at 100hrs), (e) 8YSZ

sample (MCaO/MV2O5 : MMgO/MV2O5 =1:2 at 100hrs), (f) 8YSZ sample

(MCaO/MV2O5 : MMgO/MV2O5 =2:1 at 100hrs)…………………….………………..120

Figure 4.36: XRD analysis of CaO-MgO-inhibitive vanadium-induced hot corrosion 8YSZ

powder samples with varying CaO/V2O5 and MgO/V2O5 molar concentrations

at 900oC isothermal heating. (i) Sample with MCaO/MV2O5 : MMgO/MV2O5 =1:1,

xxi

(ii) Sample mixture with MCaO/MV2O5 : MMgO/MV2O5 =1:2, (iii) Sample mixture

with MCaO/MV2O5 : MMgO/MV2O5 =2:1. (a) As-received 8YSZ, (b) 2hrs heating,

(c) 5hrs heating, (d) 20hrs heating,(e) 50hrs heating, (f)100hrs heating…………123

Figure 4.37: Amount of tetragonal zirconia phase transformation against time for CaO-MgO-

inhibitive vanadium-induced hot corrosion 8YSZ powder samples at 900oC

isothermal heating. (8YSZ) 8YSZ powder, (1MgO:2CaO) Sample

with MMgO/MV2O5:MCaO/MV2O5=1:2, (2MgO:1CaO) Sample with

MMgO/MV2O5:MCaO/MV2O5=2:1, (1MgO:1CaO) Sample with

MMgO/MV2O5:MCaO/MV2O5=1:1, (10% V2O5) 8YSZ+V2O5 (10% wt. 8YSZ)…….124

Figure 4.38: Amount of tetragonal zirconia transformation comparison of 8YSZ samples with

V2O5 conccentration (10% wt. 8YSZ) at 100hrs-900oC isothermal heating in

vanadium-induced hot corrosion inhibitive experiments using different

inhibitors………………………………………………………………………….125

Figure 4.39: SEM micrographs of CaO-MgO-inhibitive vanadium-induced hot corrosion 8YSZ

powder samples at 100hrs-900oC isothermal heating. (i) Sample with

MCaO/MV2O5 : MMgO/MV2O5 =1:1, (ii) Sample with MCaO/MV2O5 : MMgO/MV2O5 =1:2,

(iii) Sample with MCaO/MV2O5 : MMgO/MV2O5 =2:1……………………………..128

xxii

Figure 4.40: EDS analysis of rough-face spherical shaped powder particles in CaO-MgO-

inhibitive vanadium-induced hot corrosion sample

(CaO/V2O5: MgO/V2O5 =1:1)…………...……………………………………….129

Figure 4.41: EDS analysis of powder particles in CaO-MgO-inhibitive vanadium-induced hot

corrosion sample (CaO/V2O5: MgO/V2O5 =1:2)…………..……………………..130


corrosion sample (CaO/V2O5: MgO/V2O5 =2:1)…………..……………………..131

Figure 4.43: Physical appearnce of APS samples after 100hrs-900oC isothermal inhibitive hot

corrosion test………….…………………………………………………………..135

Figure 4.44: XRD analysis of APS samples after 100hrs-900oC isothermal heating inhibitive hot

corrosion test. (i) CaO-inhibitive test, (ii) MgO-inhibitive test. (a) Sample A,

(b) Sample B, (c) Sample E, (d) Sample F, (e) Sample C, (f) Sample D………...136

Figure 4.45: Amount of tetragonal zirconia phase transformation in the APS TBCs samples after

100hrs-900oC isothermal heating inhibitive hot corrosion test…………...………137

Figure 4.46: XRD analysis of as-received powders and slurry powders after inhibitive hot

corrosion test. (a) As-received V2O5 powder, (b) As received CaO powder,

(c) Slurry powder of CaO-V2O5 with MCaO/MV2O5=3, (d) Slurry powder of CaO-

xxiii

V2O5 with MCaO/MV2O5=5, (e) As receieved MgO powder, (f) Slurry powder of

MgO-V2O5 with MMgO/MV2O5=3, (g) Slurry powder of MgO-V2O5 with

MMgO/MV2O5=5……………..……………………………………………………..138

Figure 4.47: Comparison of the inhibitive efficiency of alkaline-earth metal oxides in inhibiting

vanadium-induced hot corrosion of 8YSZ. (i) Vanadium-induced hot corrosion

of 8YSZ powder samples with V2O5 concentration (10% wt. 8YSZ),

(ii) APS 8YSZ TBC samples……….…………………………………………….140

Figure 5.1: Lewis acid-base preferential reactions of compounds……….…………………….143

Figure 5.2: Volume fraction of m-ZrO2 produced in vanadium-induced hot corrosion 8YSZ

powder tests as a function of V2O5 concentration and time……………….……….144

Figure 5.3: (i) MgO-inhibitive experiment , (ii) CaO-inhbitive experiment. (a) As-received 8YSZ

powder, (b) Isothermal heated 8YSZ powder, (c) Vanadium-induced hot corrosion

sample, (d) Sample of M(alkaline earth metal oxide/vanadium oxide)=1, (e) Sample

of M(alkaline earth metal oxide/vanadium oxide)=2, (f) Sample of

M(alkaline earth metal oxide/vanadium oxide)=3 (g) Sample of

M(alkaline earth metal oxide/vanadium oxide)=5……………...…………………………………154

Figure 5.4: CaO-MgO inhibitive experiment (a) As-received 8YSZ, (b) Isothermal heated 8YSZ,

xxiv

c) Vanadium-induced hot corrosion 8YSZ, (d) Sample with MCaO/MV2O5 :

MMgO/MV2O5 =1:1, (e) Sample with MCaO/MV2O5 : MMgO/MV2O5 =1:2,

(f) Sample with MCaO/MV2O5 : MMgO/MV2O5 =2:1…………….……………………155

Figure 5.5: Flow chart on the use of alkaline-earth metal oxides to inhibit vanadium-induced hot

corrosion of 8YSZ………………………………………………………………….156

xxv

LIST OF TABLES

Table 1.1: Utilities fuel oil 310CST product specification………………………………………10

Table 1.2: Peak intensities of phases in vanadium-induced hot corrosion of 8YSZ powder

sample…………………………………………………………………….………….18

Table 1.3: Products obtained from MgO-V2O5 solid reaction at 1010oK……………..………....23

Table 2.1: Hot corrosion of YSZ, YTaO4-ZrO2 and TaO5-ZrO2 in NaVO3 and V2O5 at 700oC

and 900oC isothermal heating……………………………………………..…………26

Table 2.2: Hot corrosion resistant of ceria-stabilized zirconia……………………….………….31

Table 2.3: Inhibition Efficiency in the presence of Mg(OH)2 for vanadium-induced hot corrosion

of SA-178 specimen………………………………………………………………….44

Table 3.1: Powder samples and their source……………………………………………………..49

Table 3.2: Percentage weight compound composition of 8YSZ powder (Metco 204NS)……....49

Table 3.3: The effect of isothermal heating in hot corrosion test………………………………..53

Table 3.4: Vanadium-induced hot corrosion experiment with varying V2O5 concentration….....55

Table 3.5: Inhibitive hot corrosion experiment with MgO powder inhibitor……………………55

Table 3.6: Inhibitive hot corrosion experiment with CaO powder inhibitor………….…………56

Table 3.7: Inhibitive hot corrosion experiment with mixture of CaO and MgO powders as

inhibitor………………………………………………………………………………56

Table 3.8: Inhibitive hot corrosion experiment with lower V2O5 concentration….…..…………57

Table 3.9: APS 8YSZ-TBCs samples……………………………………………………………57

Table 4.1: Colour scale of 8YSZ powder with varying V2O5 concentration hot corrosion

xxvi

samples at 900oC isothermal heating……….………………………………………..71

Table 4.2: Colour scale of MgO-inhibitive vanadium-induced hot corrosion 8YSZ powders

samples with varying MgO/V2O5 molar concentration(MMgO/MV2O5) at 100-

900oC isothermal heating………...…………………………………………………..79

Table 4.3: Colour scale of MgO-inhibitive vanadium-induced hot corrosion of 8YSZ+V2O5

(5% wt. 8YSZ)+MgO samples with varying MMgO/MV2O5…………..………………89

Table 4.4: Colour scale of powder particles for CaO-inhibitive hot corrosion samples of 8YSZ+

V2O5 (10% wt. 8YSZ)+CaO with varying MCaO/MV2O5……………………………100

Table 4.5: Colour scale of powder particles for the CaO-inhibitive hot corrosion experiments of

mixture of 8YSZ, V2O5(2% wt. 8YSZ) and CaO………..…………………………113

Table 4.6: Colour scale of powder particles for CaO-MgO inhibitive vanadium-induced hot

corrosion 8YSZ powder samples.………………………….……………………….122

Table 4.7: Inhibition efficiency of using alkaline-earth metal oxides to inhibits vanadium-

induced hot corrosion of 8YSZ powder…………………………………………….139

Table 5.1: Comparison of monoclinic zirconia volume fraction of 2hrs-900oC isothermal

heated 8YSZ cooled via laboratory air and furnace……….………………………..141

Table 5.2: Magnesium vanadate compounds and their melting points……………...………….145

Table 5.3: Calcium vanadate compounds and their melting points…………………………….147

Table 5.4: Physical appearance of 8YSZ-APS of the coated samples at 100hrs……………....153

xxvii

THESIS ABSTRACT (ENGLISH)

NAME: GBADAMOSI ALIYU ARISEKOLA

TITLE: Evaluation of the Effectiveness of Alkaline-earth metal oxides in

Inhibiting Vanadium-induced Hot Corrosion of Yttria-stabilized Zirconia

Thermal Barrier Coatings (TBCs)

MAJOR FIELD: MECHANICAL ENGINEERING

DATE: December, 2013

Vanadium-induced hot corrosion is an accelerated high temperature corrosion of yttria stabilized

zirconia (YSZ) ceramic topcoat in Thermal Barrier Coatings (TBCs) of gas turbine engines. It

occurs due to YSZ interaction with fused vanadium salt that results from the usage of fuel

contaminated with vanadium. The hot corrosion causes spallation and degradation of the topcoat

due to reaction of yttria stabilizer (Y2O3) with the vanadium salt based on Lewis-acid mechanism

and presence of molten acidic salts; and transformation of the yttria stabilized zirconia from

tetragonal zirconia phase to monoclinic.

The use of alkaline earth-metal oxides (MgO and CaO) to inhibit vanadium-induced hot

corrosion of YSZ has been investigated experimentally using powder and as-sprayed samples.

Samples with alkaline-metal/vanadium-oxide molar concentrations of 1, 2, 3 and 5 were

subjected to 900oC isothermal heating for up to 100hrs and subsequent cooling in laboratory air.

xxviii

Characterizations by colour spectrum, XRD, SEM, EDS and X-ray mapping analyses were

carried out to estimate the phase distribution. Samples with low molar concentrations of 1 and 2

indicated degradation and destabilization of the 8YSZ within short period of heating while those

with high molar concentration ratios showed inhibition effect of vanadium-induced hot

corrosion. CaO showed better inhibition when compared to MgO of the same molar

concentration to V2O5. Samples with MgO/V2O5=5, CaO/V2O5=3 and CaO/V2O5=5 molar

concentrations displayed high inhibition efficiencies. The mechanism of inhibition is found to be

consistent with Lewis-acid mechanism.

Master of Science Degree

King Fahd University of Petroleum and Minerals

Dhahran, Saudi-Arabia

December, 2013

xxix

THESIS ABSTRACT (ARABIC)

ملخص الرسالة

: االسم عهيى اسيسكىال ججبدايىس

تقييى فبعهيخ اكبسييذ انعبد االسظيخ انقهىيخ ف تخجيػ انتبكم نهطالء انقبوو نهحشاسح انصع ي انضسكىيب :عنوان الرسالة

انستقش ثبنيبتيشيب

انهذسخ انيكبيكيخ : المجال الرئيسى

2013سجتجش : تاريخ الدرجة

نطالء انسيشاييك انصع ي انضسكىيب انستقش (عذسدسجبد انحشاسح انعبنيخ)انتبكم انستحج ثبنفببديىو يعجم انتبكم

.ثبنيبتيشيب نهطالء انقبوو نهحشاسح ف تشثيبد انحشكبد انغبصيخ

عه وهزا يحذث تيجخ ان تذاخم انضسكىيب انستقش ثبنيبتيشيب يع يصهىسايالح انفببديىو انبتجخ ع استخذاو وقىد يحتىي

يسجت انتبكم افصبل وتذهىس انطالء انعهىي ويشجع رنك ان تفبعم اكسيذ انيبتيشيب يع ايالح انفببديىو وانقبئى عه . انفببديىو

وتحىل انضسكىيب انستقش ثبنيبتيشيب ي انضسكىيب سثبع انجهىسح ان : انيخ حط نىيض ووجىد يصهىس ايالح االحبض

.احبدي انجهىسحانضسكىيب

نتخجيػ انتبكم انحج ثبنفببديىو (اكسيذ انبجييضيىو واكسيذ انكبنسيىو) تى دساسخ استخذاو االكبسييذ انعبد االسظيخ انقهىيخ

اكسيذ انفببديىو ثتشكيضاد /انعيبد يع يعبد قهىيخ. نهضسكىيب انستقش ثبنيبتيشيب يعهيب ثبستخذاو انجىدسح كعيبد يتششح

xxx

تى عم تىصيف . يئىيخ نذح يبئخ سبعخ حى تى انتجشيذ ف انهىاء داخم انعم 900 يىل تى تعشيعهب نحشاسح 5و4و3و2و1

.ثبستخذاو غيف انىا واشعخ اكس انجهش االنكتشو و تحهيم انعبصش نحسبة تىصيع االغىاس

و عذو استقشاس انضسكىيب انستقش ثبنيبتيشيب خالل يذح صييخ قصيشح تبكم 2و1 انىن االقم اظهشد انعيبد راد انتشكيض

ايعب اظهشد انتبئج . ثيب انعيبد راد انتشكيض انىالسي انعبن اظهشد تخجيػ جيذ نهتبكم انستحج ثبنفببديىو . ي انتسخي

اظهشد تبئج انعيبد ثتشكيض . ا اكسيذ انكبنسيىو نه تخجيػ جيذ يقبسخ ثبكسيذ انبجيضيىو نفس انتشكيض ي اكسيذ انفببديىو

وانكسيذ انكبنسيىو 3 وسيخ اكسيذ انكبنسيىو نالكسيذ انفببديىو ثسجخ 5ثسجخ (اكسيذ انبحسيىو نالكسيذ انفببديىو )يىن

.وتى انتىصم ان ا انيخ انتخجيػ يتىافقخ يع انيخ حط نىيس. اظهشد كفبئخ تخجيػ عبنيخ 5نالكسيذ انفببديىو ثسجخ

دسجخ انبجستيش ف انعهىو

جبيعخ انهك فهذ نهجتشول وانعبد

انهكخ انعشثيخ انسعىديخ , انظهشا

2013ديسجش

.

1

CHAPTER 1

INTRODUCTION

1.1 Coatings

Coatings are defined as any protective films applied to the surface of a material. McGraw-Hill

Dictionary of Scientific and Technical dictionary defines coatings as thin films of material

bonded to metals in order to add specific surface properties, such as corrosion or oxidation

resistance, color, attractive appearance, wear resistance, optical properties, electrical resistance

and thermal protection. It can be inferred from various definitions, that coatings are used to

improve the aesthetic, impermeability, wetability and adhesion properties of a substrate. It also

improves corrosion, wear and scratch resistance of a substrate. A substrate is a parent or base

material that is designed for a particular function in a system on which surface a coating is

applied. Coatings are majorly used for protecting the substrate to elongate its life span.

1.2 Types of Coatings

Coatings are classified into various groups based on the following criteria, which includes

operating temperature of the system, mechanism of coating deposition and coating materials etc.

(i). Temperature of the System

Birks et al. [1] classified coatings into ambient temperature coatings and high temperature

coatings. Ambient temperature coatings include paints for car corrosion protection and those for

jewelry aesthetic appearance; they are specifically for systems operating within room

2

temperature. High temperature coatings include diffusion coatings, overlay coatings and thermal

barrier coatings (TBCs) that are used in systems operating at high temperature.

(ii). Method of Coatings

Coatings are classified as overlay or diffusion coatings based on their bonding interaction with

the substrate. Overlay coatings are those that are deposited on the substrate while diffusion

coatings are those which diffuse into the substrate forming chemical bonds [1, 2]. Diffusion

coatings diffuse into the substrate during deposition to form part of the substrate. It includes

aluminide, chromized and silicide coatings obtained from elements of aluminum, chromium and

silicon, respectively. The major disadvantage of diffusion coatings is the limitation in elements

that can be used as diffusion coatings. However overlay coatings give better versatility in the

number of elements that can be part of the coatings. It does not form bond with the substrate but

forms a layer on top of the substrate. It is often an alloy of metals and includes Ni-Cr-Al and Co-

Cr-Al base coating alloys.

(iii). Coating Materials

Roberge [3] classified coatings into three general types namely organic, metallic and inorganic

coatings based on the coating material. Figure 1.1 shows various types of coatings based on their

parent material composition.

(iv). Coating Applications

Coatings are also classified based on purpose of application on a substrate. Aesthetic coatings are

used for aesthetic purpose like improving the facial appearance of the substrate by making it

more appealing and beautiful. It includes coatings for jewelries and enameling for ceramics.

3

Paints for automobiles and buildings can also be classified as aesthetic coatings. Wear, corrosion

and scratch coatings are the major applications of coatings; such coatings protect the substrate

from the effects of wear, scratch and corrosion. It includes TBCs in hot section of gas turbine

engines and boilers, galvanize steel, paints in automobiles and marines

Other applications of coatings are fire proof, water proof, anti-foulings and sound proof

applications.

Figure 1.1: Types of Coatings [4]

1.3 Thermal Barrier Coatings (TBCs)

Thermal barrier coatings (TBCs) are protective coatings for thermal insulation in high

temperature applications [1, 2] because TBCs can achieve temperature gradient reduction of

between 150oC to 175

oC across the coating [1, 5]. TBCs find applications in gas turbine engine

blades, vanes, seals and combustor chambers of aircraft, marines and land base power generation

plants; and in boilers [1, 5-7]. Engine components in high temperature applications e.g. gas

4

turbines engines and boilers, are mostly made from metal alloys that have temperature limits.

Aside the metal alloys temperature limits, engines are also subjected to higher operating

temperature due to required high efficiency; this necessitates the use of thermal barrier coatings

to protect these metal alloy substrates. A good example is the gas turbine engine blades that are

made from nickel based alloys which are designed for strength but have high temperature limits.

The TBCs in conjunction with internal air cooling provide an optimum operating temperature for

nickel based alloy blades in gas turbine engines as shown in Fig. 1.2 [1, 6, 7]. TBCs consist of a

ceramic topcoat and metal alloy bondcoat [1, 2, 5, 8]. The ceramic topcoat provides the

insulative protection for the substrate [1, 2, 7, 8]. Although historically the earlier forms of

ceramic topcoats in TBCs were alumina, calcium oxide stabilized zirconia and magnesium oxide

stabilized zirconia [1], insulative ceramic topcoat presently used in TBCs is yttria stabilized

zirconia (YSZ) [1, 5, 9, 10]. Zirconia compound constitute larger composition of the present

generation of ceramic topcoat used in TBCs due to high thermal coefficient of expansion

comparable to substrate metal alloys and low thermal conductivity properties which made it an

ideal ceramic topcoat material [2, 11]. The property of zirconia also includes its existence in

different crystal structures, at room temperature as monoclinic and at higher temperature as

tetragonal and cubic zirconia [7]. The existence of different phase of zirconia makes it undergo

phase transformation phenomenon when temperature changes from high to room temperature

and vice versa. The most commonly referenced transformation of zirconia compounds is that of

transforming from the tetragonal phase to monoclinic phase which is accompanied with 3-5%

increase in volume of the zirconia compound [11]. Yttria (Y2O3) like other oxides is used to

stabilize the zirconia compound to hinder this transformation with temperature variation; in

which 7-8 wt.% yttria has been proven as an ideal stabilizer for tetragonal zirconia in TBCs [1, 5,

5

7, 9, 10]. Fig.1.3 shows the phase diagram of ZrO2 and Y2O3. The bondcoat which is either

cobalt or nickel based alloys; MCrAlY, (M=Ni, Co) protects the substrate from high temperature

oxidation by forming a thermally grown oxide (TGO) layer of Al2O3 [1, 2, 7, 8]. Al2O3 is a

known stable oxide at high temperature; the bondcoat also provides a good adherence of the

ceramic topcoat with the substrate [1, 2, 12].

1.4 Methods of TBCs Deposition

There are two major methods of depositing YSZ-TBCs on the substrate. Bondcoat and ceramic

topcoat are deposited by either air plasma spray (APS) or electron beam physical vapour

deposition (EB-PVD) [1, 2, 7, 8, 13]. APS has been seen as a simple technique and it gives the

ceramic topcoat a splat-like structure with high porosity. EB-PVD unlike APS produces a

columnar structures [1, 13]. EB-PVD topcoat has smoother surface finish, longer thermal cycle

lifetime, better surface finish retention and superior erosion resistance when compared to APS

topcoat. Fig. 1.4 shows YSZ ceramic topcoat deposited by APS and EB-PVD methods.

1.5 Hot Corrosion

Hot corrosion is defined as an accelerated high temperature corrosion caused by reaction of

corrosive molten salts of sodium (Na), vanadium (V), sulphur (S) and phosphorus (P) at high

temperature with metal alloys and ceramics [14]. Hot corrosion in TBCs is associated with the

use of low grade fuels, that contain contaminant elements of vanadium (V), sodium (Na), sulphur

(S) and phosphorus (P) [8, 9, 15-18]. Low grades of marine distillate fuels are allowed up to 0.05

weight percent ash which contains appreciable amounts of 17 wt % sodium, up to 2 wt.% sulfur

and up to 100 ppm vanadium [19]. Table 1.1 also shows the composition of a 310CST utilities

type of low grade fuel from ARAMCO with 200ppm vanadium composition. However despite

6

the impending danger of hot corrosion; there has been increase in the use of low grade fuels to

power gas turbine due to its relative availability and low cost. This is buttress by facts like, the

present crude oil world reserve indicates that only about 25% is low vanadium crude oil and

there is tendency that vanadium-free fuels will become increasing expensive in the future [20].

Gasem and Khalid [8] also reported increase in inclination in the use of low grade fuels to power

land base gas turbine in Saudi-Arabia due to its readily availability and relatively lower cost. The

use of low grade fuels to power gas turbines makes it prone to hot corrosion from molten salts

formed from the contaminants.

Figure 1.2: Thermal Barrier Coatings [7]

The combustion of these contaminants produce molten salts of V2O5, NaCl, Na2SO4 and P2O5

that initiate and cause hot corrosion of the TBCs [8]. The 8YSZ ceramic topcoat and alloy

bondcoat are subjected to degradation due to hot corrosion from these molten salts. The extent of

hot corrosion degradation depends on the type of molten salt formed by fuel contaminants

7

present, operating temperature, thermal cycle, mass composition of the molten corrosive salt,

time and presence of other forms of impurities in the environment [1].

1.5.1 Hot corrosion of 8YSZ in the Presence of P2O5 contaminant

Mohan et al. [16] reported the destabilization of an APS 8YSZ on a steel substrate in the

presence of P2O5 molten salt. The degradation occurs at lower and higher temperature due to the

reaction of the contaminant with the zirconia compound forming ZrV2O7 as shown in reaction

1.1. Mohan et al. [16] also reported that the hot corrosion mechanism involves the contaminant

leaching out the zirconia compound from the stabilized ceramic topcoat. The leaching effect

consequentially led to increase in mass composition of the stabilizer in the ceramic topcoat that

causes transformation of the zirconia phase, thus degradation occurred.

ZrO2 (s) (in YSZ) + P2O5(l) ZrP2O7(s) + c-ZrO2(s)(yttria rich) (1.1)

1.5.2 Hot corrosion of 8YSZ in the Presence of Na2SO4 contaminant

Na2SO4 as a contanminant doesn’t cause the destabilization of the 8YSZ ceramic topcoat. Mohan

et al. [16] reported the stability of the tetragonal zirconia phase of an APS 8YSZ after hot

corrosion test at 900oC after 1hr in the presence of Na2SO4 molten salt. Hiroshi et al. [21] also

reported zero degradation of an APS 8YSZ ceramic topcoat in the presence of Na2SO4 +NaCl as

contaminants when the ceramic topcoat was subjected to 1273oC isothermal corrosion test after

3hrs.

8

Figure 1.3: ZrO2-Y2O3 phase diagram [22]

1.5.3 Hot corrosion of 8YSZ in the Presence of Na2SO4+V2O5 contaminants

8YSZ ceramic topcoat however undergoes destabilisation with Na2SO4 contanminant in the

presence of V2O5. The destabilsation of an as-sprayed 8YSZ topcoat was recorded in the

presence of 50% Na2SO4 + 50% V2O5 [16] , 75% wt. Na2SO4 + 25% wt. V2O5 [8] , 85%

V2O5+15% Na2SO4 and 15% V2O5 + 85% Na2SO4 [21]. The reason was associated with the

formation of acidic sodium vanadate compound from the molten salts as shown in reaction 1.2

and Fig. 1.5. The sodium vanadate which is an acidic compound reacts with the yttria stabilizer

and causes transformation of the tetragonal to monoclinc zirconia as depicted in reaction 1.3 [8,

9

16]. The growth of YVO4 compound is also seen as part of the hot corrosion products that causes

the degradation of the topcoat.

Figure 1.4: SEM micro-structural micrographs. (a) APS YSZ,(b) EB-PVD YSZ [23]

Na2SO4 (ss) + V2O5 (ls) 2NaVO3 + SO3 (1.2)

t-ZrO2 (Y2O3) (ss) +2NaVO3 (ls) YVO4+ m-ZrO2+Na2O (1.3)

The formation of the sodium vanadate corrosive salt can also occurs via chemical reaction shown

in reaction 1.4.

Na2O (base) +V2O5 (acid) 2NaVO3 (1.4)

10

Table 1.1: Utilities fuel oil 310CST product specification [ARAMCO]

TEST GUARANTEE METHOD

Aluminum Less than 5 (Note a,c)

Sodium Max 20 (Note a)

Nickel Max 20 (Note a)

Vanadium, ppm Max 200 (Note a)

Sulphur, wt% Max 3.7 ASTM D-4294

ASTM D-1552

Ash, wt% Max 0.10 ASTM D-482

Carbon Residue, wt% Max 16 ASTM D-189

Yugeswaran et al. [24] in Fig. 1.6 shows XRD analysis of hot corrosion test of a gas tunnel

plasma sprayed 8YSZ ceramics topcoat in 40% wt. V2O5 + 60% wt. Na2SO4 after 5hrs of

isothermal heating at 1173K. The XRD analysis showed YVO4 and transformation of tetragonal

to monoclinic zirconia as the hot corrosion products. The degradation mechanism also follows

reactions 1.2 and 1.3. Ahmaniemi et al. [25] also reported the same hot corrosion products in the

same type of corrosive salt after 200hrs of isothermal heating of an APS 8YSZ at 650oC.

Habibi et al. [6] reported the destabilization of air plasma sprayed 8YSZ with bondcoat on a

nickel alloy substrate at 1050oC sprayed with 20mg/cm

2 salt solution of V2O5 + Na2SO4. The

degradation occurred via chemical reaction, with formation of YVO4 due to high mobility of Y3+

ion and its reaction with the V ion. Habibi et al. [6] reported that the reaction was accompanied

with 3-5% increase in volume of the 8YSZ topcoat due to transformation of the tetragonal

zirconia to monoclinic zirconia.

11

Habibi et al. [6] also reported increase in hot corrosion rate of the 8YSZ in V2O5+ Na2SO4 with

increase in the quantity of the salt deposition in the experiment due to availability of more

corrosive salt.

Habibi et al. went further to show the dependence of hot corrosion on thermal cycling in hot

corrosion test of 8YSZ at 1050oC isothermal heating in molten salt of V2O5+ Na2SO4. Major

spallation and destabilization of the 8YSZ ceramic was recorded after 20hrs for 5-4hrs thermal

cycle.

An air plasma sprayed 8YSZ indicates cracking at the topcoat/bond coat after 400hrs but

excessive spallation in the topcoat after 700hrs at 900oC isothermal heating when subjected to a

molten salts of 75% wt. Na2SO4 +25% wt. V2O5 purely oxidizing air environment [8]. The

growth of YVO4 and transformation of the zirconia phase were reported as the hot corrosion

products that accompanied the degradation.

Unlike others, Hiroshi et al. [21] reported the presence of YVO4 and Y8V2O17 as corrosion

products of 1273oC isothermal heating of 8YSZ in 85% wt. V2O5 + 15% wt. Na2SO4 after 3hrs.

The presence of strong concentration of cubic zirconia, medium concentration of tetragonal

zirconia and weak presence of monoclinic zirconia were also indicated as corrosion products.

Minor attack of the 8YSZ topcoat occurred when the corrosive salt was a mixture of 15% wt.

V2O5 + 85% wt. Na2SO4 after 3hrs [21].

Nakaira et al. [21] monitored the hot corrosion destabilization of the ceramic topcoat with an AE

signal. AE signals were generated at beginning of 1273oC isothermal heating of 8YSZ in molten

corrosive salts of pure V2O5, 85% wt. V2O5+15% wt. Na2SO4 and15% wt. V2O5+85% wt.

12

Na2SO4 hot corrosion tests. Fig.1.7 shows the AE signals comparison between 8YSZ without

corrosive salt, with corrosive salt of 85% wt. V2O5+15% wt. Na2SO4 and unstabilized zirconia

ceramic topcoat. The unstabilized zirconia indicates high AE count due to large destabilization of

the zirconia phase and the 8YSZ without corrosive show the least count due to less

transformation of the zirconia compound.

Figure 1.5: V2O5-Na2SO4 phase diagram [25]

13

Figure 1.6: Hot corrosion of 8YSZ in the presence of Na2SO4+V2O5[24]

Figure 1.7: AE signal monitoring of destabilization of ceramic topcoat[21]

14

1.5.4 Hot corrosion of 8YSZ in the presence of V2O5 contaminant

Vanadium contaminant in low grade fuels oxidizes to V2O5 salt at high temperature that causes

vanadium-induced hot corrosion in YSZ-TBCs. V2O5 has melting point of 698oC which makes it

to be in molten form at high operating temperature in gas turbine engines. Its presence in molten

form in gas turbine expedites its infiltration into the porous ceramic layer TBCs to initiate and

cause vanadium-induced hot corrosion. Fig. 1.8 shows the TGA and DSC plots for V2O5

showing its melting point. Vanadium-induced hot corrosion of YSZ ceramic topcoat in TBCs

occurs via dissolution, chemical reaction and infiltration [11]. The hot corrosion mechanism

involves dissolution and chemical reaction in the planar reaction zone (PRZ) which is the top

layer of the ceramic topcoat where the molten salt deposits [11]. The mechanism via dissolution-

chemical reaction and infiltration occurs in the melt infiltration reaction zone (MIRZ) which is

in-depth of the ceramic topcoat [11]. Fig. 1.9 shows various zones in ceramic topcoat of YSZ

TBCs.

Likewise stabilized zirconia reaction mechanism due to vanadium-induced hot corrosion occurs

via chemical reaction or mineralization depending on the zirconia stabilizing oxide [13]. CeO2

and In2O3 stabilized zirconia ceramic topcoat vanadium-induced hot corrosion occur via

mineralization. The hot corrosion products via mineralization mechanism are the stabilizing

oxide itself and the monoclinic zirconia phase. Vanadium-induced hot corrosion of YSZ and

Sc2O3 stabilized zirconia occur via chemical reaction of the stabilizer with the V2O5 molten salt.

Vanadium-induced hot corrosion via chemical reaction is based on Lewis acid mechanism [10,

20], where there is preferential reaction between strong acid and strong alkaline in presence of

other weak acids and bases. In 8YSZ ceramic topcoat, vanadium-induced hot corrosion involves

the contaminant V2O5; the acid leaching out the Y2O3 stabilizer that is of high basicity compared

15

to the ZrO2 compound. The leaching of the stabilizer brings forth the transformation of the

tetragonal zirconia phase to monoclinic phase and formation of YVO4 compound. The zirconia

phase transformation causes about 3-5% increase in its volume that initiates cracks and spallation

of the topcoat. Reaction 1.5 shows the vanadium-induced hot corrosion reaction equation for

8YSZ ceramic topcoat in TBCs.

t-ZrO2 (Y2O3) (ss) + V2O5 (ls) YVO4 + m-ZrO2 (1.5)

Zun Chen et al. [10] showed that products of vanadium-induced hot corrosion of an APS yttria

stabilized zirconia depend on the isothermal heating temperature and time. In-situ XRD

monitoring of phase evolution of YSZ topcoat at 700oC and 750

oC in the presence of V2O5

indicates the presence of ZrV2O7 and YVO4 phases at initial stage of heating. Zun Chen et al.

[10] reported the transformation of the ZrV2O7 to monoclinic zirconia after some minutes of

isothermal heating at these temperatures due to instability nature of the ZrV2O7 compound. At

800oC and 900

oC, the corrosion products from the beginning of the experiment were the YVO4

and m-ZrO2 due to transformation of the t-ZrO2 [10]. Figure 1.10 shows phase evolution against

time at different temperatures for the in-situ XRD monitoring of vanadium-induced corrosion of

7% wt. Y2O3-ZrO2. Reactions 1.6 to 1.8 show the reaction equations and corrosion products of

vanadium-induced hot corrosion of the 7% wt. Y2O3-ZrO2 topcoat at different temperatures.

t-ZrO2 (Y2O3) (ss) +V2O5 (ls) YVO4+ m-ZrO2 (1.6)

ZrO2 (in t-ZrO2) +V2O5 (liquid) ZrV2O7 (1.7)

Y2O3 (in t-ZrO2) +V2O5 (liquid) YVO4 (1.8)

16

Figure 1.8: TGA and DSC plot for V2O5[26]

Figure 1.9: Hot corrosion zones in TBCs[11]

17

Figure 1.10: Phase evolution versus time plot for vanadium oxide hot corrosion of 7YSZ ceramic

topcoat [10]

Nakira et al. [21] like [10] reported the degradation of an APS 8YSZ on a 304 type stainless steel

substrate after 3hr-1273oC isothermal heating in V2O5. The degradation occurred via growth of

YVO4 and transformation of the zirconia which is a similitude of the destabilization of 7YSZ at

intermediate temperature of 900oC reported by [10].

Mohan et al. [16] also validates the fact that products of vanadium-induced hot corrosion of

8YSZ ceramic topcoat are dependent on the temperature. At temperature above 747oC the

mechanism of destabilization is the leaching of the stabilizer to form YVO4 and transformation

of the zirconia phase as shown in reaction 1.5. The hot corrosion products below this temperature

are mixture of ZrV2O7 and t-ZrO2 as shown in reaction 1.9.

ZrO2 (in YSZ) + V2O5 ZrV2O7 + t-ZrO2 (YSZ) (1.9)

18

The mass composition of V2O5 also determines the rate of destabilization of YSZ ceramic

topcoat. The rate of evolution of monoclinic zirconia from tetragonal zirconia for an APS 7YSZ

sprayed with 1% wt. V2O5 is greater when compared to 0.1% wt. V2O5 added as corrosive salt

[10].

Kondos [27] showed the effect of different concentration of vanadium oxide on the rate of

destabilization of 8YSZ using powder samples. Kondos [27] showed that larger concentration of

vanadium oxide contaminant hastens the rate of vanadium-induced hot corrosion and produced

large destabilization of the ceramic compound. Table 1.2 gives summary of peak intensities of

the tetragonal zirconia, monoclinic zirconia and YVO4 phase with different concentration of

V2O5.

Table 1.2: Peak Intensities of phases in vanadium induced hot corrosion of 8YSZ powder

sample[27]

19

The ultimate effect of hot corrosion of YSZ topcoat that arises as a result of change in volume of

the zirconia compound due to transformation are spallation, cracks and degradation of the

ceramic topcoat.

1.6 Reaction of Calcium oxide (CaO) and Magnesium oxide (MgO) with V2O5

CaO and MgO are oxides of alkaline earth metals of calcium (Ca) and magnesium (Mg)

respectively. Alkaline earth metals occupies group 2 in the period table. Mg occupies period 3 on

the periodic table; it has atomic mass and number of 24.305 and 12 respectively. Ca occupies

period 4, it has atomic mass and number of 40.078 and 20 respectively. MgO is a white

hydroscopic solid with melting point and boiling point of 2852oC and 3600

oC respectively. It has

a spinel crystal structure and density of 3.58g/cm3. CaO also referred to as quicklime is also a

white hydroscopic solid with melting point and boiling point of 2572oC and 2850

oC respectively.

It has a density of 3.35g/cm3. The reactions between the oxides of calcium and magnesium with

vanadium oxide give metallic vanadate compounds. The vanadate compounds formed depend on

the molar concentration of the reacting oxides and temperature as shown in the phase diagram in

Figs. 1.11 and 1.12. High molar concentration of the alkaline oxide reaction with vanadium

oxide i.e. 3:1 molar concentration ratio; yields vanadate compounds with high melting point,

Mg3V2O8 and Ca3V2O8 as depicted in the phase diagrams. MgV2O6 and CaV2O6 are reaction

products when lower molar concentration of the alkaline oxides reacts with vanadium oxide. The

reaction of intermediate molar concentration of the alkaline oxides produces Mg2V2O7 and

Ca2V2O7 respectively as shown in the phase diagrams.

Clark and Morley [28] carried out an experimental research on solid-to-solid reaction of MgO

and V2O5 with different molar concentration of MgO. The mixture were heated at 1010oK,

20

870oK and 770

oK for 14hrs in a furnace and characterized with DTA and XRD. Table 1.3 shows

various vanadate compounds formed from different molarity combinations of MgO-V2O5 solid

reaction at 1010oK. The chart indicates the formation of two phases of Mg2V2O7, namely alpha-

Mg2V2O7 and beta-Mg2V2O7 which are not mentioned in the MgO-V2O5 phase diagram.

Figure 1.11: MgO-V2O5 phase diagram [29]

Said and Wahab [30] reported the formation of Mg3V2O8 in the reaction between the products

from decomposition of Mg(OH)2 and ammonium metavanadate calcined at 400oC,500

oC and

700oC respectively. Said and Wahab [30] reported MgO and V2O5 as the initial products formed

from the decomposition of the two compound which reacts to give magnesium vanadate

compounds as the final product. Mg3V2O8 formed when the molar concentration of ammonium

21

vanadate is between 20% to 30% but MgV2O6 formed when the its molar concentration reaches

50% in the reaction.

Miyauchi et al. [29] in the production of vanadium metal by Preform Reduction Process

produced Ca2V2O7 and Mg2V2O7 by mixing slurry mixtures of V2O5-CaO and V2O5-MgO

respectively as initial reaction products. Metal vanadates compound of CaVO4 and MgVO4 were

final products from the initial vanadate compounds after reduction with Ca and Mg vapour

respectively.

Figure 1.12: CaO-V2O5 phase diagram [29]

22

1.7 Motivation and Objective

1.7.1 Motivation

The motivation for this research arose due to increase in inclination for the use of low grade fuels

in land base gas turbine in Saudi Arabia. Low grade fuels are readily available and cheap. Based

on the literature reviewed, fuel additives method appeared to be the easiest, less costly and most

suitable alternative for inhibiting vanadium-induced hot corrosion in gas turbines. Most literature

have worked on fuel additives for inhibiting vanadium-induced hot corrosion in metal alloys

with little or none on TBCs. This research is meant to delve into the suitability of using oxide

additives in inhibiting vanadium-induced hot corrosion of yttria stabilized zirconia topcoat in

TBCs of turbine engines. Most of the methods proposed to inhibit the hot corrosion haven’t been

hundred percent effective and these methods are complex to incorporate into the TBCs. The use

of oxide additives will be the most viable option in inhibiting vanadium-induced hot corrosion of

TBCs considering its simplicity and cost. MgO and CaO which are the proposed inhibitors are

readily available and cheap; aside this is the simplicity behind the technology of achieving the

inhibition goal.

1.7.2 Objective

To evaluate and compare the effectiveness of magnesium oxide (MgO) and calcium oxide

(CaO) as additives to inhibit vanadium-induced corrosion of 8YSZ in Thermal Barrier Coatings

(TBCs).

23

Table 1.3: Products Obtained from MgO-V2O5 solid reaction at 1010oK [28]

Molar concentration

ratio of

MgO:V2O5/Magnesium

Vanadate compounds

7:2 3:1 5:2 2:1 3:2 1:1 2:3

Mg3V2O8 1 1 1

α- Mg2V2O7

β- Mg2V2O7 3 1 1 1

MgV2O6 1 1 1

MgO 2

V2O5 2

Key to symbols: 1, major constituent; 2, minor constituents (>10%); 3, trace constituents (˂10%)

24

CHAPTER 2

LITERATURE REVIEW

Inhibition of vanadium-induced hot corrosion of 8YSZ ceramic topcoat in TBCs involves

different methodologies which are basically aimed at preventing the acid-base reaction and

transformation of the stabilized zirconia associated with temperature change. The methods

include seeking alternative stabilizers that are less reactive with the vanadium oxide to stabilize

the zirconia compound. Methods of preventing infiltration of the molten salts into the ceramic

topcoat by sealing mechanism and sacrificial overlay coating have been examined; and the use of

alternative ceramic topcoat such as pyrochlores has also been explored. The use of additives for

inhibiting vanadium-induced hot corrosion has also been reported extensively for metal alloys at

high temperature, though its use for ceramic topcoat has received less attention.

2.1 Alternative Stabilizers for zirconia compound

Vanadium-induced hot corrosion of YSZ topcoat occurs via Lewis acid mechanism, which

necessitates the technology of seeking alternate stabilizers that are of less basicity compared to

yttria. This is aimed at retarding the acid-zirconia stabilizer reaction thereby hindering the

destabilization of zirconia; phase transformation reduces if the leaching of zirconia stabilizer by

reaction with V2O5 is hindered. CeO2, Ta2O5 and Sc2O3 etc. have been found as better

alternatives to yttria because they are of lesser basicity. This reduces their affinity and reaction

with the acidic vanadate salts, hence hot corrosion of the topcoat is reduced. Some of these

alternate stabilized zirconia ceramics are commercially available, while some are not; but their

vanadium-induced hot corrosion resistances have been evaluated:

25

2.1.1Ta2O5-stabilized Zirconia

Ta2O5 is an oxide of tantalum that has either hexagonal or orthorhombic crystal structure and has

a melting point of 1872oC. The hot corrosion resistant of Ta2O5 stabilized zirconia has been

quantified by using a tetragonal phase YTaO4-ZrO2 obtained from 10% mol Y2O3 and 10% mol

Ta2O5; and an orthorhombic Ta2O5-ZrO2 (14TZ) obtained from 14% mol Ta2O5 [31]. YTaO4-

ZrO2 like YSZ has low thermal conductivity, better phase stability at elevated temperature,

comparable co-efficient of thermal expansion, lower elastic modulus and ability to be deposited

by plasma spray, while 14TZ has low thermal conductivity [31].

In pure V2O5 environment isothermal heating, at lower and intermediate temperature above the

melting point of V2O5; 700oC and 900

oC using powder samples, 14TZ powder showed resistance

to hot corrosion. YTaO4-ZrO2 however under the same condition undergoes degradation due to

leaching of yttria and corresponding precipitation of 14TZ rather than mass growth of the

monoclinic zirconia [31].

In pure NaVO3 contaminant, under the same experimental conditions YTaO4-ZrO2 powder

undergoes little or minute degradation with weak signal of YVO4, monoclinic zirconia and

NaTO3. This was deduced to reduction in activity and diffusibility of Y3+

ion due to presence of

Ta5+

in the stabilized zirconia. 14TZ in the same contaminant at 700oC showed resistance to

degradation like YTaO4-ZrO2, but at 900oC it undergoes destabilization forming monoclinic

zirconia and sodium tantalite [31].

The hot corrosion resistance of the two forms of tantalum oxide stabilized zirconia at 700oC and

900oC in NaVO3 and V2O5 molten salt compared to the YSZ is presented in the Table 2.1. The

result indicates the two forms of tantalum stabilized zirconia are more resistant to hot corrosion

26

to vanadium salts than YSZ, but YTaO4-ZrO2 is more resistant to NaVO3 out of the two; while is

vice versa for resistant to V2O5.

Table 2.1: Hot corrosion of YSZ, YTaO4-ZrO2 and Ta2O5-ZrO2 in NaVO3 and V2O5 at 700oC

and 900oC isothermal heating [31]

2.1.2 Sc2O3- stabilized Zirconia (SSZ)

Scandia (Sc2O3) is an amphoteric white solid oxide of scandium with melting point of 2485oC.

Scandia has a stable oxidation state of 3+, is the most acidic of the rare earth oxides and 5%

scandia is sufficient to stabilize tetragonal zirconia [20]. Thermo gravimetric study of scandia

oxide powder reaction at 800oC with molten NaVO3 under SO3 partial pressure showed it to be a

promising hot corrosion resistant zirconia stabilizer. It showed growth of ScVO4 at higher SO3

partial pressure compared to the growth of YVO4 with Y2O3 at lower partial pressure under the

same experimental conditions [32].

In furtherance to this, the reactions of scandia powder with NaVO3 and V2O5 at 700oC and

900oC, characterized with XRD show formation of ScVO4 with the two melt at 700

oC, but was

only evident with V2O5 melt at 900oC [20]. The study shows the suitability of scandia as an

27

alternate stabilizer for zirconia that will be of better resistance than yttria to hot corrosion from

fuel impurities of vanadium, sulphur and sodium.

Jones [20] showed the superiority of scandia stabilized zirconia to yttria stabilized zirconia using

5%wt. scandia stabilized zirconia pellet subjected to isothermal heating at lower and

intermediate temperature above the melting point of V2O5. At 700oC with thin film of NaVO3;

SSZ depicts no transformation of phase present in the as-received pellet sample while YSZ

undergoes transformation and YVO4 evolving from the process under the same test condition

[20]. SEM micrograph after 475hrs of 700oC isothermal heating is shown in Fig. 2.1. It indicates

the formation of rod-like structure YVO4 in YSZ, unlike SSZ that shows no formation of new

phases. XRD characterization analysis of SSZ pellet after 160hrs of 900oC isothermal heating in

vapour deposition of NaVO3 is shown in Fig. 2.2 compared with that of YSZ pellet under the

same test conditions. The SSZ pellet maintains its structure of SSZ at 900oC after 160hrs

isothermal heating compared to the YSZ pellet which has its yttria leached out to form YVO4

and has transformed from tetragonal zirconia to monoclinic zirconia as shown in the XRD

analysis [20].

Figure 2.1: SEM micrograph of YSZ and SSZ after 475 hrs isothermal heating at 700oC. (a)

YSZ, (b) SSZ [20]

28

Figure 2.2: XRD analysis of YSZ and SSZ after 160 hrs isothermal heating at 900oC [20]

2.1.3 MgO-stabilized Zirconia (MSZ)

Magnesium oxide stabilized zirconia (MSZ) is one of the earliest insulative ceramic topcoats in

TBCs [1]. It is commercially available presently from Sulzer Company. Like YSZ with between

7-8% wt. yttria stabilizer, MgO stabilized zirconia with these compositions have been reported

22%wt. MgO-ZrO2 [25], 7.6%wt. MgO-ZrO2 [21] and 15-30%wt. MgO-ZrO2 (Sulzer Company).

Jones [32] performed thermo gravimetric analysis of MgO as a stabilizer for zirconia at 800oC

with molten NaVO3 under SO3 partial pressure. The result obtained was formation of MgSO4

and Mg3V2O8 as weight gain at lower SO3 partial pressure, which was similar to formation of

YVO4 for Y2O3 stabilizer also at lower SO3 partial pressure. The reaction for MgO and Y2O3

occurred at lower partial pressure compare to formation of ScVO4 and InVO4 from Sc2O3 and

In2O3 respectively at higher SO3 partial pressure under the same experimental conditions. This

study portrays MgO as similar to Y2O3 when used to stabilize zirconia which was further proven

by other hot corrosion researches on magnesium oxide stabilized zirconia.

29

Jones [13] reported that the reaction equations for degradation of magnesia stabilized zirconia in

a burner rig test at 800oC with a fuel doped with various levels of vanadium, phosphorus,

sodium, calcium, iron and magnesium etc. follow reactions 2.1 and 2.2.

ZrO2 (MgO) + SO3 (g) ZrO2 (monoclinic) + MgSO3 (2.1)

ZrO2 (MgO) + V2O5 (l) ZrO2 (monoclinic) + Mg3V2O8 (2.2)

The reaction in equation 2.1 represents the destabilization of the magnesia stabilized zirconia

when magnesium is presence in the fuel why that of equation 2.2 represents the reaction

occurring when magnesium is absent in the fuel. Like yttria stabilizer, MgO stabilizer was

leached out by the vanadium oxide and sulphur oxide contaminant to form magnesium vanadate

and magnesium sulphate respectively and is accompanied by phase transformation of the

zirconia compound to m-ZrO2.

Ahaminiemi et al. [25] carried out an experimental test on destabilization resistance of an as

sprayed magnesium oxide stabilized zirconia in the presence of 60% Na2SO4 + 40% V2O5 at

650oC isothermal heating for 200hrs. XRD analysis of the hot corrosion sample indicates large

presences of m-ZrO2 peaks as a result of the transformation of the cubic 22%wt. MgO-ZrO2 as-

spayed specimen.

At higher temperature Nakira et al. [21] reported the attack of APS 7.6MgO-ZrO2 on a 304 type

stainless steel substrate subjected to 1273oC isothermal heating in the presence of vanadium

oxide for 3hrs. The specimen showed spallation and cracks after the hot corrosion test.

30

Although thermgravimetric analysis by Jones [32] indicates similarity between MgO and yttria

stabilizer, but the probable superiority of magnesia stabilized zirconia to YSZ in vanadium oxide

was presented in the reaction in eq. 2.3 by [13].

ZrO2 (Y2O3) + Mg3V2O8 (l) ZrO2 (monoclinic) + MgO + YVO4 (2.3)

2.1.4 CeO2-stabilized Zirconia (CSZ)

CeO2 is an oxide of rare earth metal of cerium. It has a melting point and boiling point of 2400oC

and 3500oC respectively. It has a density of 3.75g/cm

3. CSZ is cerium oxide stabilized zirconia,

its tetragonal in crystal structure, it has a much lower thermal conductivity and a higher thermal

expansion coefficient than YSZ [33]. CSZ is commercial available from Sulzer Company.

Cerium stabilized zirconia has been proven to have better resistant to hot corrosion compare to

YSZ. The hot corrosion resistant of ceria-stabilized zirconia was evaluated using CSZ stabilized

with 10%, 12.5% and 15% mol of CeO2 [34]. The test was conducted at 950oC isothermal

heating with Na2SO4 and a mixture of Na2SO4 plus 2% mol of V2O5 as the molten salt

respectively. Table 2.2 summarizes what was observed after different hours of isothermal heating

at 950oC from the XRD characterization of phase that evolved from the test. The result indicates

higher transformation of tetragonal zirconia to monoclinic zirconia when 2% mol of V2O5 is

contained in the molten salt compare to pure Na2SO4 and the higher the ceria composition the

lower the transformation. It also confirms that ceria stabilized zirconia destabilize in the presence

of V2O5.

Park et al. [33] investigated hot corrosion resistance of zirconia stabilized with 25% wt. CeO2

and 2.5% wt.Y2O3[CYSZ]. The as-sprayed CSYZ on a Ni-alloyed substrate was subjected to

31

900oC isothermal heating for 300hrs after adding salt sample of NaVO3 to the specimens. XRD

and SEM characterization methods indicate mineralization of CeO2 and consequential growth of

the monoclinic zirconia and YVO4 phase after 70hrs of heating. At 300hrs of isothermal heating,

CeVO4 phase developed and complete transformation of the tetragonal phase to monoclinic

phase accompanied with cracks and spallation due to volume change occurred. Figure 2.3, shows

XRD analysis of hot corrosion test of the CYSZ specimen.

Table 2.2: Hot corrosion resistant of ceria-stabilized zirconia [34]

Molten salt 10%CeO

2-ZrO2

10%CeO

2-ZrO2

12.5%CeO

2-ZrO2

12.5%CeO

2-ZrO2

15%CeO

2-ZrO2

15%CeO

2-ZrO2

% m-

ZrO2

% t-ZrO2 % m-ZrO2 % t-ZrO2 % m-

ZrO2

% t-ZrO2

As-sintered

CeO2-ZrO2

2 98 1 99 2 98

Na2SO4 20hr

s

7 93 3 97 1 99

Na2SO4+2%m

ol V2O5

5hrs 75 25 25 75 10 90

Transformation of tetragonal zirconia to monoclinic zirconia, mineralization of CeO2 to form

CeVO4 and leaching of yttria to form YVO4 was also the result of the hot corrosion test of

CeO2/Y2O3 stabilized zirconia in Na2SO4 +V2O5 environment [35].

Nakahira et al. [21] also reported attack of an as sprayed 15% CeO2-ZrO2 in the presence of

V2O5 when subjected to 1273oK isothermal heating for 3hrs. In the presence of

32

85%V2O5+15%Na2SO4 under the same test condition, minor attack of the ceramic topcoat was

observed.

The tests all depict CeO2-ZrO2 is of higher hot corrosion resistant when compare with a full YSZ

because the CeO2 stabilizer is more acidic than yttria and based on Lewis acid mechanism; the

V2O5 reaction with CeO2 is slower compared to its leaching effect on yttria.

Figure 2.3: XRD analysis of hot corrosion test of zirconia stabilized with 25% wt. CeO2 and

2.5% wt. Y2O3[33]

2.1.5 CaO-stabilized zirconia (CaO-ZrO2)

CaO stabilized zirconia (CaO-ZrO2) like MgO-ZrO2 was one of earliest stabilized form of

zirconia topcoats in TBCs [1] and it is commercially available from Sulzer Company. Like 7-

8YSZ, calcium oxide stabilized zirconia with 10CaO-ZrO2 composition was reported by [21].

33

CaO-ZrO2 resistant to vanadium-induced hot corrosion is lesser when compared to YSZ because

it is of more basicity than Y2O3, the CaO stabilizer reacts easily with the molten salt of

vanadium.

Nakahira et al. [21] reported cracks and spallation features in an as-sprayed 10CaO-ZrO2 on steel

substrate subjected to1273oC isothermal heating for 3hrs in the presence of V2O5 only and

85%V2O5+15%Na2SO4. Spallation and broken features appeared under the same test condition

for the same as-sprayed calcium oxide stabilized zirconia but in the presence of

15%V2O5+85%Na2SO4 and 90%Na2SO4+10%NaCl respectively. Figure 2.4 shows the features

observed for the CaO-ZrO2 in different corrosive molten salt.

Figure 2.4: SEM micrograph of Hot Corrosion of 10CaO-ZrO2 in molten fuel contaminant salts

[21]

2.1.6 In2O3-stabilized zirconia

India (In2O3) is an amphoteric oxide of indium with melting point of 1910oC and it has cubic

crystal structure. In2O3 has high vapour pressure that makes it vaporizes out of zirconia when

deposited on substrate by APS; but EB PVD deposition of 3.5-11% mol In2O3 stabilized cubic

zirconia was reported [36].

34

In2O3 like Sc2O3 in thermo gravimetric study of stabilizing oxides of zirconia reactions at 800oC

with molten NaVO3 in SO3 partial pressure atmosphere indicates formation of InVO4 compound

at higher SO3 partial pressure [32]. Both oxides showed higher hot corrosion resistant trend when

compared to that of reaction between the molten salt and yttria oxide to form YVO4 [32].

Vanadium-induced hot corrosion resistant of india stabilized zirconia powder was tested in an

environment of NaVO3 with various amount of V2O5 melts [36]. The 3.9% In2O3-ZrO2 powder

undergoes destabilization in pure environment of NaVO3 with less composition of V2O5 melt

forming In2O3, but formation of InVO4 was observed with higher concentration of V2O5 melt. It

was concluded that destabilization occurs by mineralization at lower concentration of V2O5 melts

but by chemical reaction at higher concentration, though proved to be better when compared to

YSZ [36].

2.1.7 SnO2-stabilized zirconia

Tin oxide (SnO2) is a white powder with melting point of 1630oC and it has tetragonal crystal

structure. Though tin oxide stabilized zirconia is not commercially available, but it has been seen

as a promising ceramic topcoat with better resistance to hot corrosion. SnO2 is inert to Na2O-

V2O5-SO3 attack [36]. XRD characterization of reaction between tin dioxide with probable

oxides composition of impure fuel containing S, V and Na in a combustible environment; at

700oC and 800

oC indicates no new phases emerging [19]. This confirm the non-reactivity of tin

oxide with these fuel contaminant molten salts at high temperature and depicts tin oxide either as

a component or been alloy with other compound will resist hot corrosion of molten salts. These

properties of tin oxide also prove tin oxide to be an excellent material for stabilizing zirconia for

35

hot corrosion resistant [19, 36], hence tin oxide stabilized zirconia will be more hot corrosion

resistant at lower temperature [36].

2.1.8 TiO2-stabilized zirconia

Titania (TiO2) is a white solid oxide of titanium with melting point of 1842oC and boiling point

of 2972oC. TiO2 can exists in form of rutile, anatase which both has tetragonal crystal structure

and it also exists in orthorhombic and monoclinic crystal forms [18]. Like tin oxide, titania oxide

has also been seen as a promising zirconia stabilizer, though titania stabilized zirconia is not

commercially available. TiO2 is a moderate acidic, it neither reacts with NaVO3 nor V2O5 but

undergoes mineralization like CeO2, changing from anatase to rutile structure but it reacts with

Na2SO4 easily forming NaTiO3 and SO3 [36]. The transformation of tetragonal or cubic structure

to monoclinic phase due to mineralization was observed for ceramic mixture of TiO2-ZrO2 in a

vanadate corrosion test [36].

2.1.9 Yb2O3 stabilized Zirconia (Yb2O3-ZrO2)

Ytterbia is an oxide of ytterbium; it is white solid with melting point of 2355oC. It is more acidic

when compared to yttria and can be use effectively to stabilized zirconia [36]. The activity of

V2O5 at 900oC using thermodynamic data required to form YbVO4 is 9.3X10

-10 which is higher

than that required to form YVO4 (8X10-11

) but much more less when compare to that required to

form ScVO4 [36]. Yb2O3 stabilized zirconia resistance to vanadate will not have much difference

when compared to that of YSZ [13].

The alternate stabilized zirconias haven’t been proven to be the best alternative to yttria

stabilized zirconia in mitigating vanadium-induced hot corrosion. The alternate stabilized

36

zirconias that are commercially available though retard the degradation but still undergo

degradation in the presence of molten salts. Others which have promising better resistant are

only available on paper not in practice.

2.2 Pyrochlores

Pyrochlores (Rare earth zirconates) are composite compound of zirconia and lanthanide metals

with chemical formular M2Zr2O7, where M represents the lanthanide metal. Pyrocholores have

been seen as an alternative to 8YSZ as ceramic topcoat in TBCs due to its better resistance to hot

corrosion and other desirable properties. Pyrochlore has low thermal conductivity, high

temperature stability, high thermal co-efficient of expansion and better stability to accommodate

defects [6].

Sa Li et al. [37] evaluated the hot corrosion resistant of rare earth zirconates of

ytterbium,Yb2Zr2O7 in V2O5. The plasma sintered pyrochlore undergoes destabilization at 600oC

isothermal heating at both 2hrs and 8hrs forming compounds of YbVO4 and ZrV2O7. Yb2Zr2O7 at

700oC formed YbVO4 with either m-ZrO2 or ZrVO7 after 8hrs and 2hrs respectively. YbVO4 and

m-ZrO2 formed mainly both after 2hrs and 8hrs isothermal heating at 800oC due to instability of

the ZrVO7 at higher temperature.

Yugeswaran et al. [24] evaluated La2Zr2O7 pyrochlore hot corrosion resistant in molten salt of

40%V2O5+60%Na2SO4 at 1173oK isothermal heating after 5hrs. La2Zr2O7 is rare earth zirconate

of lanthanum. The gas tunnel plasma sprayed lanthanum zirconate gave a better hot corrosion

compared to 8YSZ, though it undergoes destabilization forming LaVO4 and tetragonal-

monoclinic zirconia.

37

Gd2Zr2O7 hot corrosion resistance in molten salt mixture of Na2SO4 and V2O5 was evaluated by

Habibi et al. [6]. An APS Gd2Zr2O7 subjected to 1050oC isothermal heating developed spallation

and cracks after 9cycles of 4hrs of heating. The pyrochlore hot corrosion products are GdVO4

and m-ZrO2. An APS 8YSZ under the same test condition suffered spallation and cracks after

5cycles.

Like the alternate stabilized zirconias, the use of pyrochlores has also been seen not to provide

hundred percent mitigation of hot corrosion. It only retards the phenomenon of hot corrosion in

the ceramic topcoat in presence of molten salts.

2.3 Ceramic Composites

Ceramic composite topcoats in TBCs are made from mixing together two promising ceramic

topcoat materials aimed at achieving a desirable property i.e. better hot corrosion resistant. 8YSZ

that is the most prominent ceramic topcoat material is often mixed with other ceramic materials

like alumina and pyrochlore for improved hot corrosion resistant. Hot corrosion resistant of such

ceramic topcoat composites have also been evaluated as a means of achieving a durable ceramic

topcoat. Yugeswaran et al. [24] carried out an experiment to evaluated plasma spray composite

ceramic topcoat of 50% 8YSZ+50% La2Zr2O7 hot corrosion resistant in NaVO3 at 1173oK

isothermal heating for 5hrs. The formation of flake like LaVO4 inhibits formation of YVO4 that

made the hot corrosion resistance of composite better compared to 8YSZ ceramic topcoat.

Habibi et al. [6] also evaluated composite mixture of Gd2Zr2O7 +8YSZ hot corrosion resistant. It

showed better resistance compared to 8YSZ which undergoes destabilization after 5cycles of

4hrs isothermal heating. GdVO4 growth from the composite inhibits the formation of YVO4

38

which prolongs the spallation time of the composite till after 9cycles of 4hrs heating at 1050oC

isothermal heating.

Afrasiabi et al. [9] showed that the rate of transformation of tetragonal zirconia to monoclinic

zirconia in an APS YSZ is greater than when having APS composite mixture of 60%

wt.YSZ+40%wt.Al2O3. In the presence of NaVO3, after 40hrs of hot corrosion test carried out at

1050oC; APS YSZ indicates the presence of 66% m-ZrO2 while the composite indicates 20% m-

ZrO2. Although both indicate spallation and cracks; that of the composite is less severe compared

to the APS YSZ. Fig. 2.5 shows the volumetric monoclinic zirconia in different types of APS

ceramic topcoats.

Figure 2.5: Volume fraction of monoclinic zirconia in the coatings after hot corrosion test, (a)

YSZ, (b) YSZ+Al2O3, (c) YSZ/Al2O3[9]

2.4 Overlay coating

The use of overlay coatings which are often sacrificial layers have also been employed to

mitigate hot corrosion problem in TBC. Alumina based composites have been the major forms of

overlay coatings used for protecting TBC. Xiaolong et al. [38] evaluated the effectiveness of an

overlay coating alumina composite to mitigate hot corrosion in TBCs. LaMgAl11O19 coating on

a b c

39

an APS 8YSZ was subjected to 900oC isothermal heating in the presence of V2O5. Its resistant

was evaluated and compared to an APS 8YSZ under the same experimental conditions.

Characterization of the overlay coating TBCs specimen after 50hrs indicates the growth of YVO4

which is the same for the free APS 8YSZ specimen at 20hrs. The overlay coating undergoes

destabilization to protect the ceramic topcoat by hindering infiltration of the molten salt into it.

8YSZ layer of the overlay coated specimen showed longer destabilization life span since it is not

in direct contact with the contaminant compared to the free APS 8YSZ specimen that comes in

contact with V2O5 directly.

Afrasiabi et al. [9] showed that the presence of an APS alumina layer on an APS YSZ on nickel

alloy substrate offered greater resistance to destabilization in NaVO3. The hot corrosion test

XRD analysis after 40hrs at 1050oC isothermal heating indicates the presence of considerable

amount of t-ZrO2 compared to that of an APS YSZ tested under the same experimental

conditions. The better resistant offered by the APS YSZ with alumina overlay coat is attributed

to the reduction in the infiltration of molten salt into it by the alumina layer.

Mohammadreza et al. [39] obtained lesser degradation of an APS YSZ when protected with a

nano-Al2O3 structure as an overlay coating in presence of V2O5+Na2SO4 molten salt. The

transformation of the zirconia phase and growth of YVO4 after 52hrs heating at 1000oC was

reduced when compared with that of an APS YSZ under the same test conditions. The nano-

Al2O3 covered YSZ gave a better hot corrosion resistance due to the nano-Al2O3 overlay

hindering the infiltration of the contaminant into the topcoat hence reduces its accelerated

corrosion.

40

Z. Chen at al. [40] also reported the reduction in destabilization of an APS 8YSZ topcoat covered

with an EB-VPD alumina layer. The growth of YVO4 and transformation of the zirconia phase

reduced when compare with an APS 8YSZ in direct contact with molten salt of

95%Na2SO4+5%V2O5 heated at 950oC between 10mins to 100hrs. The rate of the destabilization

of the TBCs was calculated using equation 2.4, where M and T represents the intensity of the

monoclinic (-111) and tetragonal (111) zirconia peaks from XRD analysis. Figure 2.6 shows the

comparison between the two topcoats, with the YSZ/alumina system showing less destabilization

out of the two; though both portray increase in destabilization with time. The alumina layer

reduces the infiltration of the molten salt into 8YSZ ceramic topcoat, hence making it more

resistance to hot corrosion.

(2.4)

Figure 2.6: Rate of Destabilization versus time plot of as sprayed ceramic topcoat[40]

41

Canumalla et al. [41] evaluated the effectiveness of three forms of overlay coating in protecting

the TBCs against hot corrosion of molten salt of Na2SO4. The alumina composites include

mullite (3Al2O3.2SiO2) and BAS (BaO.Al2O3.2SiO2), while the third is calcium silicate

compound (1.8CaO.SiO2). The overlay coatings were air plasma sprayed on an APS 8YSZ TBC

and all the specimens subjected to 900oC isothermal hot corrosion test in molten Na2SO4. Their

hot corrosion resistant were compared to that of an APS 8YSZ under the same experimental

condition. Although the contaminant doesn’t react with YSZ but its solidification after

infiltration caused destabilization of the topcoat. Mullite proved to be best out of the overlay

coatings neither does it reacts nor allows the infiltration of the molten salt. The 8YSZ for the

calcium silicate coatings undergoes destabilization due to reaction of the calcium silicate with

the contaminant forming a new compound. The formation of new compound generates stress

mismatch between the YSZ/overlay coating boundary which initiates the destabilization of the

topcoat, though still portrays more resistant than free 8YSZ.

2.5 Surface Sealing

Hot corrosion of 8YSZ especially APS type, is hasten due to infiltration of molten salt into the

in-depth of the ceramic topcoat through the porosity. Hindering the infiltration of the molten salt

through various methods has been seen as an alternative way to mitigate hot corrosion of 8YSZ

ceramic topcoat in TBCs often by laser glazing.

Batista et al. [42] carried out an experimental analysis on mitigation of hot corrosion in ceramic

topcoat with laser glazing. Nd-YAG and CO2 laser glazed APS 8YSZ on a steel substrate

resistant to hot corrosion from V2O5+Na2SO4 contaminant was evaluated and compared with that

of APS 8YSZ. The specimens were subjected to 1000oC isothermal heating and subsequent

42

cooling in open air. The laser glazed specimens show no sign of destabilization up till 72hrs as

compared to APS 8YSZ in V2O5 and combination of V2O5 and Na2SO4 contaminants, however

the APS 8YSZ specimen shows the growth of YVO4 within 24hrs of isothermal heating. Hot

corrosion mitigation in the laser glazed specimens was attributed to reduced porosity of the

ceramic which hinders infiltration of the contaminants. The little destabilization observed in the

laser glazed was due to the infiltration of molten salt through the lateral pores created by the

laser glazing operation. Fig. 2.7 shows the laser glazed surface with lateral pores and the

unglazed ceramic topcoat with high level of porosity.

Figure 2.7: SEM surface morphology of ceramic topcoat before hot corrosion,(a) as sprayed ,(b)

CO2 laser glazed, (c) Nd-YAG laser glazed [42]

Ahmaniemi et al. [25] evaluated 60%Na2SO4+40%V2O5 molten salt hot corrosion resistant of

different types of ceramic topcoat sealing methods. 8YSZ and 22MgO-ZrO2 ceramic topcoats

sealed by phosphate base sealant, Nd-YAG laser glazing and dense spraying using detonation

gun hot corrosion resistant was compared to that of as-sprayed 8YSZ and 22MgO-ZrO2 ceramic

topcoats. The specimens were subjected to the same hot corrosion test of 650oC isothermal

heating for 200hrs and characterization with XRD, SEM after cooling in open air. Nd-YAG

topcoat destabilization took longer hours compared to that of other sealed surfaces, but the

43

degradation is worst in the un-sealed ceramic topcoats. The MgO stabilized zirconia topcoat also

posed higher hot corrosion resistant in the contaminant environment compared to the 8YSZ

topcoat. The hot corrosion resistances of the sealed surfaces were attributed to reduction in the

porosity of the topcoat which reduces the infiltration of the molten contaminant.

2.6 Fuel Additive Inhibitors

The use of fuel additives to mitigate vanadium-induced hot corrosion has been explored mostly

for metal alloys, with little on ceramic topcoats in TBCs. It is the most favourable in term of

cost and simplicity compared to other methodologies. Hot corrosion mitigation in metal alloys

using fuel additives is also based on Lewis acid mechanism and Flood Lux acid mechanism,

which define reaction between basic oxide and acidic oxide. The use of metal base compound of

Mg, Ca, Y and Ni as the fuel additives to mitigate hot corrosion of metal alloy has been

investigated and their effectiveness evaluated.

Barbooti et al. [43, 44] evaluated the efficiency of mitigating vanadic ash induced hot corrosion

of three types of steel material with Mg base compound fuel additives; magnesium sterate and

magnesium hydroxide. The inhibitive tests were conducted in the presence of vanadic ash with

67%wt. of V2O5 and 33%wt. of Na2SO4 composition in ratio 1:1, 1:2 and 1:3 to the Mg-base

inhibitors at various temperatures between 550oC and 950

oC. Maximum inhibiting efficiency

was obtained at lower temperature of 550oC and with inhibitor-vanadic ash molar ratio of 3:1. It

was attributed to the formation of high melting point Mg3V2O8 compound which peak was

indicated in XRD characterization. Fig. 2.8 and Table 5 show the effect of the magnesium based

fuel additives with different molar concentration in mitigating vanadium-induced hot corrosion

of two different type of steel materials.

44

Figure 2.8: Action of Magnesium Stearate Addition on Weight gain for (213T1) steel at 1:1,

dotted line; 2:1, dashed line and 3:1,solid [44]

Table 2.3: Inhibition Efficiency in the presence of Mg(OH)2 for vanadium-induced hot corrosion

of (SA-178) specimen [43]

Chassagneux and Thomas [45] evaluated the protection of AISI 310 steel from V2O5 hot

corrosion using Mg coating in molar concentration ratio of 3, 2 and 1 to 1 of vanadium

respectively. Weight change for the coated specimen in which Mg/V=3 gave weight change

45

equal to that obtained from oxidized specimen without molten salt which is the best. In V2O5

environment, the AISI 310 recorded a weight gain of 2.77mg.cm-2

while in the environment with

Mg/V=3, AISI 310 recorded a weight gain of 0.27mg.cm-2

. The coated specimen with Mg/V=2

and 1 gave a weight change which is 2 and 3.5 multiple that obtain for Mg/V=3 respectively.

XRD characterization also shows the presence of Mg3V2O8, Mg2V2O7 and MgV2O6 for Mg/V

equal to 3, 2 and 1, respectively.

Rhys-Jones et al. [17] evaluated the effectiveness of MgO additives in inhibiting vanadium-

induced hot corrosion of Ni and Co alloy at 750oC and 850

oC. The effectiveness of the additives

to inhibit vanadium-induced hot corrosion of metal alloy in presence of SO2/SO3 in the system

was also evaluated. The experiment involved samples of Ni and Co base alloys subjected to

100hrs; 850oC and 750

oC isothermal heating with MgO/V2O5 molar concentration equals 3. The

inhibitive effect was higher in the absence of the sulphur oxide due to the formation of high

melting point of Mg3V2O8. The presence of sulphur compound however reduces the inhibitive

effect due to MgO inhibitor reacting with the sulphur compound. Figure 2.9 shows the weight

change versus mass/area composition of the nickel alloy inhibitive hot corrosion test.

Rocca et al. [15] compared the effectiveness of fuel additives inhibitor oxides of Ni and Mg to

mitigate vanadium- induced corrosion of metal alloy. The analysis involves weight measurement

of nickel base alloy using in-situ thermogravemetric analysis subjected to isothermal heating test.

The inhibitive effect of NiO was attributed to the formation high melting point Ni3V2O8 similar

to Mg3V2O8 obtained for MgO in vanadium oxide environment. Figure 2.10, shows weight

change of the nickel base alloy with NiO and MgO additives lowered than that without the

inhibitors

46

Figure 2.9: Effect of contaminants and inhibitor on high temperature corrosion of Nimonic 90 at

850oC [17]

Singh et al. [18] used Y2O3 as an inhibitor for high temperature corrosion due to molten salt of

Na2SO4 and V2O5 for Fe, Ni and Co base alloys. The alloys were subjected to 50cycles, 900oC

isothermal heating with 1hr heating and 20minutes cooling in open air and weight change

measure during each cycle. The results showed weight change and spallation of alloy for

inhibitive tests without inhibitor is more compared to those with inhibitor as seen in Fig 2.11.

47

Figure 2.10: Weight change versus time (A) Na/V=1.5, (B) Na/V=1.5,Mg/V=3 and (c)

Na/V=1.5,Ni/V=2.25 oxidation at 850oC[15]

Figure 2.11: Weight change versus number of cycles [18]

48

CHAPTER 3

EXPERIMENTAL PROCEDURES

The use of as-sprayed YSZ (APS or EB-PVD) on metal alloy substrate specimens have been

mostly reported from the literature in hot corrosion laboratory test of TBCs [8, 10]. The TBCs

specimen is usually rectangular shaped TBCs on a metal alloy substrate coupon. The rectangular

coupon is a replicate of the turbine blade which allows for laboratory experiments to depict the

actual field experience in the gas turbine engines. Likewise the use of 8YSZ powder sample for

hot corrosion experiment has also been reported, [27, 31] as an alternative to the as-sprayed

specimen. The use of 8YSZ powder sample in hot corrosion experiment is simple and the easiest.

It also provides a means of actually ensuring homogenous mixing of the 8YSZ powder with the

contaminants and inhibitors. This gives a similitude of the molten salt infiltration in real field

experience in gas turbine engines. The objective of this research was accomplished by carrying

out an inhibitive isothermal hot corrosion experiments with powder samples and afterward using

the APS 8YSZ coupons.

3.1 Materials

The materials used in the inhibitive hot corrosion research are 8YSZ (Metco 204NS), MgO, CaO

and V2O5 powder samples. It also includes TBCs coupons; an APS 8YSZ-MCrAlY (Ni-22%Cr-

10%Al-1%Y) TBCs on IN718 nickel super-alloy substrate. The choice of 8YSZ and V2O5

samples were based on simulating the actual field experience in the gas turbine engines where

the topcoat is an as-sprayed 8YSZ and V2O5 is the molten salt formed from the combustion of

49

the vanadium contaminant at high temperature. Tables 3.1 and 3.2 below show the powder

samples and their source; and the compositional property of the 8YSZ powder, respectively.

3.2 Equipment

The equipment used in the inhibitive hot corrosion research experiment are:

a) Phoenix AB 224 weighing Device: The equipment can measure up to four decimal figures. It

was used to measure the required amount of powder samples for the experiments and weighing

the APS TBCs specimens. This is shown in Fig. 3.1(a).

Table 3.1: Powder samples and their source.

Powder Source

8YSZ (Metco 204NS) Sulzer Company

MgO Nano-scale Materials Inc.

CaO Fisher Scientific Company

V2O5 Fluka 94720 Company

Table 3.2: Percentage weight compound composition of 8YSZ powder (Metco 204NS)

Compound ZrO2 Y2O3 Al2O3 Fe2O3 TiO2 SiO2 Monoclinic(max)

%wt.

composition

Balance 7-9 0.2 0.2 0.2 0.7 10

b) Mortar, Alumina crucibles and Glass tubes: The powder samples were mixed to form

homogenous samples by mixing in the glass tubes and grinding in the mortar. The alumina

crucibles were used as containers for the portion samples of the homogenous powder samples

50

during heating in the furnace. The mortar, alumina crucibles and the glass tubes are shown in Fig

3.1(b).

c) Linderberg Blue-M Furnace: Hot corrosion in TBCs occurs at high temperature inside the gas

turbine engines, the furnace was used to subject the samples to 900oC isothermal heating for the

stated heating period of between 10minutes to 100hrs.

3.3 Inhibitive Hot Corrosion Experiment

3.3.1 Inhibitive Hot Corrosion Experiment (Powder Samples)

The powder samples experimental design was meant to simulate the worst scenario that is

obtainable in the field. This justifies the amount of the 8YSZ and V2O5 powders used in the

experiments. Kondos [27] reported 90% of 8YSZ powder with 10% V2O5 powder gave the

highest rate of degradation of the 8YSZ powder compared to those with lesser V2O5

concentration. The inhibitive tests entail varying the molar concentration ratio of the alkaline-

earth metal oxide to vanadium oxide in the homogenous samples from 1 to 5 to determine the

effective combination ratio of the powders. The powder samples inhibitive hot corrosion

experimental procedure is as follows:

i) Measuring the desired amount of powder samples for the homogenous mixture.

ii) Homogenous mixing of the powder samples in the glass tube and mortar.

iii) Distributing the homogenous mixture in small portions into the alumina crucibles before

putting it into the furnace for inhibitive hot corrosion test.

51

Tables 3.3 to 3.8 show the hot corrosion and inhibitive hot corrosion tests carried out with

different composition of the powder samples.

3.3.2 Inhibitive Hot Corrosion Experiment (APS 8YSZ-TBCs Coupon Specimen)

The experimental procedure for inhibitive hot corrosion test of the as-sprayed samples entails

firstly cleaning the APS 8YSZ specimens with an acetone-water mixture in a sonicator for

60minutes. This was subsequently followed with drying of the specimens in the furnace by

heating at 200oC for 15minutes and their weight measured. Homogenous slurry solutions of

V2O5, MgO and CaO were prepared with the following molar concentration ratio, respectively;

MMgO/MV2O5 equals 3 and 5, respectively and McaO/MV2O5 equals 3 and 5, respectively. This was

based on those alkaline-earth metal oxide-vanadium oxide molar concentration ratios that portray

good inhibitive results in the powder sample experiments. These slurry solutions were then

applied on to the surface of the specimens and assigned as samples A to F. The slurry solutions

were added gradually in each case in drops on to the surface of the as-sprayed specimen and

dried by heating until the mass weight of the slurry on each specimen was 3-5 times that of the

V2O5 powder on the vanadium-induced as-sprayed sample. The vanadium-induced as-sprayed

sample has slurry of V2O5 up to 20mg/cm2 on the surface of the specimen. The samples were

then subjected to 900oC isothermal heating for 100hrs in the furnace and subsequent cooling in

the laboratory air. Table 3.9 shows the as-sprayed samples for the inhibitive hot corrosion

experiment.

It is worthy to know that the following conditions were observed for the hot corrosion and

inhibitive hot corrosion powder samples; and APS coupon experiments:

i) Isothermal heating at 900oC

52

ii) Heating time was between 10minutes to 100hrs

iii) Vanadium contaminant was only considered (since it is the only contaminant that attacks the

ceramic topcoat).

iv) Cooling of heated samples was done in laboratory air (except when testing for isothermal

heating effect)

The powder samples experimental set-up with the alumina crucibles in the furnace is shown in

Figure 3.2(a).

3.4 Characterization Methods

Characterizations of the samples were carried out after the inhibitive hot corrosion tests in order

to explore the changes in the properties and characteristics of the samples. Characterization

methods used include the following:

i) Colour Spectrum Analysis: The change in colour of the samples during the hot corrosion test

was analyzed by visual appearance and taking pictures using camera. The use of Hunterlab

device which measures colour scale of the powder samples was also employed. Fig. 3.2(b) shows

the Hunterlab colour device.

ii) XRD analysis: XRD analysis as a method of characterization was used for phase analysis. The

growth of the monoclinic zirconia due to phase transformation of the zirconia compound from

hot corrosion was majorly monitored with the XRD analysis. Other phases of compound formed

by elemental combination of vanadium, yttrium, magnesium, calcium and oxygen were also

monitored with XRD analysis. The plot of phase intensity against 2θ was obtained from the XRD

53

analysis in which the chosen 2θ for the analysis ranges between 10o to 80

o step 0.02

o. The XRD

device used was a D8 XRD machine using a Cu Kα1 target. The D8 XRD machine is shown in

Fig. 3.3.

iii) Scanning Electron Microscopy (SEM): SEM analysis was used for morphology

characterization of the powder samples. The powder samples SEM micrographs both before and

after hot corrosion experiments were captured to monitor morphology change in the powder

samples. TESCAN LYRA machine shown in Fig. 3.4(a) and JOEL JSM-6460LV machine in

Fig. 3.4(b) were used to capture the SEM micrographs.

iv) Energy Dispersive Spectroscopy (EDS)/X-ray mapping: SEM micrographs depict only the

morphology of the powder samples, but EDS and X-ray mapping analyses gave a better insight

of the elemental distribution in the samples. In the inhibitive hot corrosion research, EDS and X-

ray mapping characterizations were used for further analyses to determine the emerging

compounds by elemental analysis. EDS analysis was carried out using the TESCAN LYRA

machine like SEM while X-ray mapping was carried out using JOEL JSM-6460LV machine

shown in Fig. 3.4(b).

Table 3.3: The effect of isothermal heating in hot corrosion test

Powder samples Weight

composition of

V2O5

Molar

concentration

ratio of

CaO:MgO:V2O5

900oC isothermal

heating time

Cooling

Mechanism

8YSZ N/A N/A 2hrs Cooling in the

furnace and

laboratory air

54

(a)

(b)

Figure 3.1: (a) Phoenix AB 224 weighing device , (b) Mortar, glass tube and alumina crucibles

55

Table 3.4: Vanadium-induced hot corrosion experiment with varying V2O5 concentration

Powder Samples Weight composition

of V2O5

900oC isothermal

heating time

Cooling Mechanism

8YSZ and V2O5 10% 8YSZ 10minutes-100hrs Laboratory air




Table 3.5: Inhibitive hot corrosion experiment with MgO powder inhibitor

Powder Samples Weight

composition of

V2O5

Molar

concentration

ratio of

MgO:V2O5

900oC isothermal

heating time

Cooling

Mechanism

8YSZ, MgO and

V2O5

10% 8YSZ 1:1 10minutes-

100hrs

Laboratory air

8YSZ, MgO and

V2O5


100hrs

Laboratory air

8YSZ, MgO and

V2O5


100hrs

Laboratory air

8YSZ, MgO and

V2O5


100hrs

Laboratory air

56

Table 3.6: Inhibitive hot corrosion experiment with CaO powder inhibitor


composition of

V2O5

Molar

concentration

ratio of

CaO:V2O5

900oC isothermal

heating time

Cooling

Mechanism

8YSZ, CaO and

V2O5


100hrs

Laboratory air

8YSZ, CaO and

V2O5


100hrs

Laboratory air

8YSZ, CaO and

V2O5


100hrs

Laboratory air

8YSZ, CaO and

V2O5


100hrs

Laboratory air

Table 3.7: Inhibitive hot corrosion experiment with mixture of CaO and MgO powders as

inhibitor


composition of

V2O5

Molar

concentration

ratio of

CaO:MgO:V2O5

900oC isothermal

heating time

Cooling

Mechanism

8YSZ, MgO,

CaO and V2O5

10% 8YSZ 1:1:1 10minutes-

100hrs

Laboratory air

8YSZ, MgO ,

CaO and V2O5


100hrs

Laboratory air

8YSZ, MgO,

CaO and V2O5


100hrs

Laboratory air

57

Table 3.8: Inhibitive hot corrosion experiment with lower V2O5 concentration

Powder Samples V2O5 weight

composition

CaO:MgO:V2O5

molar concentration

900oC isothermal

heating time

Cooling

Mechanism

8YSZ, MgO and

V2O5


100hrs

Laboratory air

8YSZ, MgO and

V2O5


100hrs

Laboratory air

8YSZ, MgO and

V2O5


100hrs

Laboratory air

8YSZ, CaO and

V2O5


100hrs

Laboratory air

8YSZ, CaO and

V2O5


100hrs

Laboratory air

8YSZ,CaO and

V2O5


100hrs

Laboratory air

Table 3.9: APS 8YSZ-TBCs samples

Sample Homogenous salt solution on

the APS 8YSZ-TBCs

specimen

Molar concentration ratio of

powder samples in the salt

solution (CaO:MgO:V2O5)

Sample A N/A N/A

Sample B V2O5 N/A

Sample C MgO + V2O5 0:3:1

Sample D MgO + V2O5 0:5:1

Sample E CaO + V2O5 3:0:1

Sample F CaO + V2O5 5:0:1

58

(a)

(b)

Figure 3. 2: (a) Inhibitive Hot corrosion experimental set-up, (b) Hunterlab colour

characterization device

59

Figure 3.3: D-8 XRD Machine

60

(a)

(b)

Figure 3. 4: (a) TESCAN LYRA Machine, (b) JOEL JSM 6460LV Machine

61

CHAPTER 4

RESULTS

The results are obtained from the characterization methods which include the colour spectrum,

XRD, SEM, EDS and X-ray mapping elemental analyses. The results are both in qualitative and

quantitative forms. Qualitative results include physical colour appearance, XRD spectra, SEM

micrographs, EDS and X-ray mapping analyses. Quantitative results are colour scale

measurement and amount of tetragonal zirconia phase transformation. The colour scale gives a

measure of the degree of lightness or blackness which is represent by L*, the degree of redness

or greenness; a* and the degree of yellowness and blueness; b* of the sample. The amount of

tetragonal zirconia transformation to monoclinic zirconia depicts the extent of accelerated

corrosion of the 8YSZ powder and APS 8YSZ topcoat; and is determine using equation 4.1. The

equation measures the volume fraction of monoclinic zirconia in the 8YSZ using data from the

XRD spectra [9, 38, 42, 46].

(4.1)

M% = volume fraction of monoclinic zirconia phase.

M1 = intensity of monoclinic zirconia phase peak; (1 1 1) plane from the XRD spectrum

M2 = intensity of monoclinic zirconia phase peak; ( , 1, 1) plane from the XRD spectrum

T = intensity of the tetragonal zirconia phase peak; (1 0 1) plane from the XRD spectrum

62

M1 and M2 are intensities of monoclinic zirconia peaks at 2θ equal 28o and 31.5

o respectively

while T is that of tetragonal zirconia peak at 2θ equals 30o.

4.1 Effect of Isothermal heating in Hot Corrosion Test

The effect of cooling rate in hot corrosion was evaluated with 8YSZ powder samples subjected

to 900oC isothermal heating for 2hrs and separately cooled in laboratory air and furnace. Fig.

4.1(i) shows the bar chart representation of monoclinic zirconia volume fractions in the samples

and the as-received 8YSZ powder using Eqn. 4.1. The volume fractions of monoclinic zirconia

for each sample of as- received 8YSZ powder, air cooled and furnace cooled appear almost the

same. This indicates that the monoclinic zirconia from the transformation of the tetragonal

zirconia is independent of the cooling rate.

The SEM micrographs for the samples are shown in Fig. 4.1(ii). The morphology of the powder

samples in both case are the same. It shows powder samples with smooth-face spherical shaped

particles. This further validates the earlier written statement that tetragonal zirconia

transformation is independent of cooling rate.

4.2 Vanadium-induced Hot Corrosion of 8YSZ Powder

4.2.1 Colour Spectrum Analysis

The effect of varying amount of V2O5 concentration on colour spectrum of 8YSZ powder hot

corrosion at 900oC isothermal heating is shown in Fig. 4.2 and Tab. 4.1. The sample with 0wt.%

V2O5 concentration; isothermal heated 8YSZ powder, Fig. 4.2(a) depicts a whitish colour at

100hrs, which is the same with the colour of the as-received 8YSZ powder. The whitish colour

of the as-received 8YSZ powder however changes with increase in heating time for sample with

63

10wt.% V2O5 concentration, Fig 4.2(b-c). The white colour turns brownish after 10 minutes of

heating, Fig. 4.2(b) and reddish after 50hrs of heating, Fig. 4.2(c). The hot corrosion products of

samples with V2O5 concentration, 2% and 5% wt. 8YSZ indicate light yellowish and dark

yellowish colour respectively throughout 100hrs heating time as shown in Fig. 4.2(d-e). The

colour scale of the samples as shown in Tab. 4.1; has the 100hrs-900oC isothermal heated 8YSZ

powder as the reference sample. The scale shows that increasing the V2O5 concentration in the

samples reduces the lightness and yellowness of the samples as shown with the isothermal heated

8YSZ sample having the highest L* and b* values while the sample with 10wt.% V2O5 shows

the lowest colour scale value for lightness and yellowness out of all the samples. The redness of

the samples however increases with increase in V2O5 concentration as shown in Tab. 4.1; sample

with 10wt.% V2O5 has the highest a* value while the sample with 2wt.% V2O5 has the lowest.

The isothermal heated 8YSZ sample portrays greenness with a negative measured a* value. The

colour scale for powder sample with 10wt.% V2O5 also shows that with increase in isothermal

heating time, there is reduction in lightness and yellowness of the sample but increase in redness

of the sample. This is shown in the colour scale for the sample at 2hrs and 100hrs of isothermal

heating.

4.2.2 XRD Analysis

Figures 4.3-4.4 show the XRD analysis of the effect of varying V2O5 concentration in vanadium-

induced hot corrosion of 8YSZ powder sample subjected to 900oC isothermal heating for 100hrs.

Fig. 4.3(i) represents the XRD spectra of 900oC isothermal heated 8YSZ powder; zero

concentration of the V2O5. The spectra which are similar to that of the as-received 8YSZ powder

spectrum Fig. 4.3(i-a), indicate the presence of only the monoclinic (PDF card number 01-037-

64

1484) and tetragonal (PDF card number 01-070-4431) zirconia phases identified with PDXL-2

software. The intensities of each phase peaks remain constant throughout the 100hrs isothermal

heating period as shown in the spectra. Fig. 4.3(ii) shows the XRD spectra for hot corrosion test

of 8YSZ powder in the presence of 2wt.% V2O5 concentration. The hot corrosion spectra unlike

those of the isothermal heated 8YSZ powder sample, aside the zirconia peaks; indicate the

presences of peaks corresponding to YVO4 compound (PDF card number 01-074-1172). The

sample indicates the presences of YVO4 peaks and monoclinic zirconia peaks at 2θ values which

are hitherto not in the as-received 8YSZ powder XRD spectrum within few minutes of

isothermal heating. It also indicates major increment of the monoclinic zirconia peaks; (111) and

( , 1, 1) planes and decrease in the tetragonal zirconia peak; (101) plane intensities within 2hrs,

which remain almost constant throughout the remaining heating period. The XRD spectra for hot

corrosion test sample with increased concentration of V2O5 (5% wt. 8YSZ) are shown in Fig.

4.4(i). It depicts the same trend with that of sample with 2wt.% V2O5; presence of the YVO4

peaks and monoclinic zirconia peaks in new 2θ values but of higher intensities at early minutes

of isothermal heating. The sample aside showing the new peaks, also shows increase in m-ZrO2

peaks; (111) and ( , 1, 1) planes intensities and corresponding decrease in that of t-ZrO2 peak;

(101) plane with the m-ZrO2 peaks outgrowing the t-ZrO2 peak at 100hrs. Fig. 4.4(ii) is the XRD

spectra for 8YSZ powder sample with 10wt.% V2O5; it has almost exactly the same features with

sample of 5wt.% V2O5 concentration. However unlike it, the t-ZrO2 peak; (101) plane intensity

reduces to zero at 50hrs of isothermal heating and has peaks of higher intensities.

Plots of m-ZrO2 volumetric fraction against time for the samples using equation 4.1 are shown in

Fig. 4.5. The values for M1, M2 and T are from the samples XRD spectra in Figs. 4.3-4.4. The

65

(i)

a b

(ii)

Figure 4.1: (i) Monoclinic zirconia volume fraction bar chart on effect of isothermal heating on

2hrs-900oC isothermal heated 8YSZ powder, (ii) SEM micrographs of 2hrs-900

oC isothermal

heated 8YSZ powder. (a) Furnace cooled sample, (b) Laboratory air cooled sample .

66

graph indicates highest volume fraction of m-ZrO2 in the sample with 10wt.% V2O5 throughout

the heating period as compared to other samples. The 10wt.% V2O5 concentration sample shows

huge m-ZrO2 volume fraction within 2hrs of heating and attains a unit value at 50hrs isothermal

heating. The isothermal heated 8YSZ powder sample shows the least volume fraction and those

samples with intermediate V2O5 concentrations (2% wt. and 5% wt. 8YSZ) depicts m-ZrO2

volume fraction which lies between these two extremes, with higher amount in the sample with

5wt.% V2O5. The plots show that major transformation of t-ZrO2 phase occurred in the early

isothermal heating time. The XRD spectra and rate of tetragonal zirconia transformation plots

indicate that high V2O5 concentration produces huge degradation of the 8YSZ powder and that

V2O5 causes vanadium-induced hot corrosion of 8YSZ ceramic topcoat in TBCs.

4.2.3 SEM and EDS Analyses

SEM micrographs of powder particles of the as-received 8YSZ and that of its 900oC isothermal

heated mixture samples with varying V2O5 concentration at 100hrs are shown in Fig. 4.6. Fig.

4.6(ii-a) shows smooth-face spherical shaped powder particles of the as-received 8YSZ powder.

The dominant presence of these smooth-face particles in the as-received 8YSZ powder spectrum

is shown in Fig. 4.6(i-a), which is an SEM of it taken at lower magnification. The SEM

micrographs of the isothermal heated 8YSZ powder are shown in Figs. 4.6(i-b) and 4.6(ii-b); it is

a resemblance of the as-received 8YSZ powder particles. It shows that no changes occurred in

the morphology of the powder particles with isothermal heating of the 8YSZ powder for 100hrs

in absence of V2O5 powder. The EDX analysis of the smooth-face spherical shaped powder

particles is shown Fig.4.7. The spectra A, B and C indicate evenly elemental distribution of

zirconium, oxygen and yttrium in the particles which is the expected elemental distribution for a

67

yttria-stabilized zirconia. Figs. 4.6i(c-e) and 4.6ii(c-e) represent the micrographs of the powder

particles for vanadium-induced hot corrosion 8YSZ powder samples with varying concentration

of V2O5; 2%, 5% and 10% wt. of 8YSZ respectively. It shows a complete transformation of the

smooth-face spherical morphology of the as-received 8YSZ particles to rough-face spherical

shaped particles; however the degree of roughness of the particles increases with increase in the

V2O5 concentration. The rough-face particles comprise a mixture of an irregular and facet shaped

structures as shown in Figure 4.6(c-e). The clearer composition of the rough-face spherical

shaped particles; irregular and facet shaped structures are shown in the backscatter/secondary

SEM micrograph taken at higher magnification as shown in Fig. 4.8(a). The analysis of these

rough-face particles using the backscatter image and EDS analysis in Fig. 4.7; spectra F and G,

indicate the irregular shape structures as zirconium and oxygen rich structures. The facet shaped

structures indicate high presence of yttrium, vanadium and oxygen elements; spectrum D and E.

The richness of the irregular shaped structures in zirconium and oxygen elements without

yttrium which is different from the elemental distribution of the as-received 8YSZ powder

particles portrays these structures as the monoclinic zirconia phase. However the presence of

majorly the yttrium element with vanadium in the facet shaped structures depicts it as the YVO4

compound formed from the reaction between Y2O3 and V2O5 as shown in the XRD spectra.

4.2.4 X-ray Mapping Analysis

X-ray mapping analysis of the rough-face spherical shaped particles in vanadium-induced

samples is shown in Fig. 4.8(b). It shows elemental distribution of oxygen, zirconium, yttrium

and vanadium in the particles. The evenly distribution of all the four element in these particles

validates the XRD, SEM and EDS results that portray the rough-face particles as composition of

68

new phases formed from zirconium, yttrium, vanadium and oxygen elements. The monoclinic

zirconia phase muddled up with YVO4 compound.

4.3 MgO-inhibition of Vanadium-induced Hot Corrosion of 8YSZ Powder with

V2O5 concentration (10% wt.8YSZ)


The colour appearance of powder samples at 100hrs of 900oC isothermal heating hot corrosion

and MgO-inhibitive hot corrosion tests is shown in Fig. 4.9. The inhibitive hot corrosion samples

are 8YSZ powder samples with varying molar concentration ratio of MgO to V2O5 powders

(MMgO/MV2O5). The sample of MMgO/MV2O5=5, Fig. 4.9(g) looks whitish and is almost a

resemblance of the isothermal heated 8YSZ sample, Fig. 4.9(a) while the sample of

MMgO/MV2O5=3, Fig. 4.9(f) appears light yellowish in colour. The sample of MMgO/MV2O5=2, Fig.

4.9(e) portrays a dark yellowish appearance but not as dark yellowish as that of sample with

MMgO/MV2O5=1, Fig. 4.9(d). The vanadium-induced hot corrosion sample, Fig. 4.9(b-c) looks

brownish and is closer in colour appearance to the sample with MMgO/MV2O5=1. Thus the higher

the MMgO/MV2O5 in the sample the closer the sample colour to that of isothermal heated 8YSZ

powder and the lower the MMgO/MV2O5 the closer colour of the sample to that of vanadium-

induced hot corrosion sample.

The colour scale for samples in Fig. 4.9 is shown in Tab. 4.2 with 100hrs-900oC isothermal

heated 8YSZ powder as the reference sample. The colour scale like in colour appearance shows

the sample of MMgO/MV2O5=5 closer to the isothermal heated 8YSZ in terms of lightness,

greenness and yellowness. The lightness and greenness of the samples however reduce with

69

reduction in MMgO/MV2O5 but yellowness increases, thus sample of MMgO/MV2O5=3 is lighter and

of less redness compared to sample with MMgO/MV2O5=2. Likewise the sample of MMgO/MV2O5=2

is lighter and of less redness compared to sample with MMgO/MV2O5=1. The colour scale also

shows that the lower the MMgO/MV2O5 in the sample the closer the lightness, redness and

yellowness of the sample to that of vanadium-induced hot corrosion sample as shown in Tab.

4.2.

4.3.2 XRD Analysis

Figures 4.10-4.11 show the XRD analysis portraying the effectiveness of using MgO powder in

mitigating vanadium–induced hot corrosion of 8YSZ powder mixture with V2O5 concentration

(10% wt. 8YSZ). The spectra represent 900oC isothermal heated samples of 8YSZ powder

mixture samples with varying MgO/V2O5 molar concentration (MMgO/MV2O5). Fig. 4.10(i) shows

the XRD spectra for sample of MMgO/MV2O5=1 between 2hrs to 100hrs. It further shows the

inhibitive hot corrosion spectra peaks deviating from that of the as-received 8YSZ powder

spectrum, with additional peaks indicating presence of YVO4 and MgV2O6 compounds (PDF

card number 00-034-0013). The sample spectra indicate the growth of YVO4 and m-ZrO2 in new

2θ positions in the early minutes of the inhibitive hot corrosion test as shown in spectrum for

2hrs. The sample likewise indicates consequential increase in the intensities of m-ZrO2; (111)

and ( , 1, 1) plane peaks and that of t-ZrO2; (101) plane peak growing vice versa. The spectra for

sample with MMgO/MV2O5=2 are shown in Fig. 4.10(ii). It bears almost the same resemblance

with that of Fig. 4.10(i), aside the presence of Mg2V2O7 (PDF card number 01-070-1163) instead

of MgV2O6 and reduced rate of change in intensities of m-ZrO2 and t-ZrO2 peaks. Figure 4.11(i)

depicts XRD spectra for sample of MMgO/MV2O5=3, just like with earlier mentioned XRD

70

analysis; m-ZrO2 and t-ZrO2 peaks are present in the spectra. However unlike that of Fig. 4.10

inhibitive spectra, the presence of the YVO4 peaks are only noticeable after 50hrs isothermal

heating and its intensity is minute. The consequential change in m-ZrO2; (111) and ( , 1, 1) and

t-ZrO2; (101) plane peaks intensities are also of minute magnitudes and Mg3V2O8 compound

(PDF card number 01-073-0207) peaks are present in the spectra as only the magnesium

vanadate compound. Mg3V2O8 compound is the expected magnesium vanadate compound from

the reaction between MgO and V2O5 with MMgO/MV2O5=3. Figure 4.11(ii) depicts the spectra for

sample of MMgO/MV2O5=5; it shows the presence of m-ZrO2, t- ZrO2 and Mg3V2O8 phases only.

The m-ZrO2 and t-ZrO2 peaks intensities also remain almost constant throughout the 100hrs

isothermal heating process just like that of the as-received 8YSZ powder spectrum.

The amount of tetragonal zirconia transformation against time plots using equation 4.1 for the

MgO-inhibitive hot corrosion, vanadium-induced hot corrosion and isothermal heated 8YSZ

powder samples are shown in Fig.4.12. The MgO-inhibitive hot corrosion test plots contain that

of 8YSZ mixture samples with MMgO/MV2O5 equal 1, 2, 3 and 5 respectively. The data for the

plots are obtained from the XRD spectra in Figs.4.10-4.11, 4.3(i) and 4.4(ii) respectively. The m-

ZrO2 volume fraction is highest in the plot for vanadium-induced hot corrosion 8YSZ sample

which is closer to that of sample with MMgO/MV2O5=1 plot. The isothermal heated 8YSZ sample

plot has the least volume fraction of m-ZrO2 followed by that of sample of MMgO/MV2O5=5. The

plots for samples with MMgO/MV2O5 equal 2 and 3 fall in between that of samples with

MMgO/MV2O5 equal 1 and 5 respectively, with that of 3 next to 5 and that of 2 next to 1.

71

Figure 4.2: Colour appearance of vanadium-induced 8YSZ powder samples with varying

concentration of V2O5 at 900oC isothermal heating. (a) Isothermal heated 8YSZ at 100hrs, (b)

2hrs isothermal heated 8YSZ+10wt.%V2O5 powder sample, (c) 100hrs isothermal heated

8YSZ+10wt.%V2O5 powder sample, (d) 100hrs isothermal heated 8YSZ+2wt.%V2O5 powder

sample, (e) 100hrs isothermal heated 8YSZ+5wt.%V2O5 powder sample

Table 4.1: Color scale of 8YSZ powder with varying V2O5 concentration hot corrosion samples

at 900oC isothermal heating

Powder samples

with varying

V2O5

concentration

L* a* b* dL* da* db*

8YSZ at 100hrs 79.86 -2.74 13.49 79.86 -2.74 13.49

8YSZ+ V2O5

(10% wt. 8YSZ)

at 2hrs

42.98 10.32 17.44 -36.88 13.05 3.95

8YSZ + V2O5

(10% wt. 8YSZ)

at 100hrs

42.02 17.99 11.43 -37.84 20.73 -2.06

8YSZ + V2O5

(5% wt. 8YSZ) at

100hrs

60.03 6.23 38.79 -19.83 8.96 25.30

8YSZ + V2O5

(2% wt. 8YSZ) at

100hrs

65.61 4.73 39.78 -14.25 7.47 26.29

72

(i)

(ii)

Figure 4.3: XRD analysis spectra (i) 900oC isothermal heated 8YSZ, (ii) 900

oC isothermal

heated 8YSZ powder sample with V2O5 concentration (2% wt. 8YSZ). (a) As-received 8YSZ, (b)

2hrs isothermal heating, (c) 5hrs isothermal heating, (d) 20hrs isothermal heating , (e) 50hrs

isothermal heating, (f) 100hrs isothermal heating

73

(i)

(ii)

Figure 4.4: XRD analysis spectra (i) 900oC isothermal heated 8YSZ powder sample with V2O5

concentration (5% wt. 8YSZ), (ii) 900oC isothermal heated 8YSZ powder sample with V2O5

concentration (10% wt. 8YSZ). (a) As-received 8YSZ, (b) 2hrs isothermal heating, (c) 5hrs

isothermal heating, (d) 20hrs isothermal heating, (e) 50hrs isothermal heating, (f)100hrs

isothermal heating

74

Figure 4.5: Amount of tetragonal-zirconia phase transformation against time for vanadium-

induced hot corrosion 8YSZ powder samples with varying V2O5 concentration at 900oC

isothermal heating. (8YSZ) 8YSZ powder, (2% V2O5) 8YSZ+V2O5 (2% wt. 8YSZ) powder

sample, (5%V2O5) 8YSZ+V2O5 (5% wt. 8YSZ) powder sample , (10%V2O5) 8YSZ+V2O5(10%

wt. 8YSZ) powder sample

75

(i)

(ii)

Figure 4.6: SEM micrographs of 100hrs-900oC isothermal heated as-received 8YSZ powder and

its mixture samples with varying concentration of V2O5. (i) Lower Magnification, (ii) Higher

Magnification. (a) As-received 8YSZ, (b) Isothermal heated 8YSZ, (c) 8YSZ+V2O5 (2% wt.

8YSZ), (d) 8YSZ+V2O5 (5% wt. 8YSZ), (e) 8YSZ+V2O5 (10% wt. 8YSZ)

76

a b

Spectrum O Y V Zr Total

A 22.9 3.7 0.0 73.4 100.0

B 26.1 4.7 0.0 69.2 100.0

C 18.6 5.3 0.0 76.1 100.0

D 11.3 37.3 29.2 22.2 100.0

E 20.9 25.3 32.3 21.5 100.0

F 21.7 0.0 1.1 77.2 100.0

G 31.9 0.0 1.9 66.2 100.0

Figure 4.7: EDS analysis. (a) Smooth-face spherical shaped powder particles, (b) Rough-face

spherical shaped powder particles

A B

A

B

C

D

77

(a)

(b)

Figure 4.8: (a) Backscatter/Secondary SEM micrograph of rough-face spherical shaped powder

particles, (b) X-ray mapping analysis of rough-face spherical shaped powder particles

(Vanadium-induced hot corrosion sample)

78

4.3.3 SEM Analysis

Figure 4.13(i-ii) are SEM micrographs of powder particles for 900oC isothermal heating of 8YSZ

powder, vanadium-induced hot corrosion 8YSZ powder with 10wt.% V2O5 and MgO-inhibitive

hot corrosion 8YSZ powder samples at 100hrs. The micrographs in Fig. 4.13-ii(a-b) show

isothermal heated 8YSZ and vanadium-induced hot corrosion 8YSZ powder samples at 100hrs

respectively. Figs. 4.13-i(a) and 4.13-ii(c) represent micrographs for the inhibitive test sample

with MMgO/MV2O5=1. It shows the smooth-face spherical shaped particles of the as-received

8YSZ powder have turned into rough-face spherical particles; a mixture of bar-shape and

irregular structures, Fig. 4.14. The micrographs in Fig. 4.13-i(b) and 4.13-ii(d) represent the

inhibitive test sample with MMgO/MV2O5=2. It shows the presence of the rough-face and few

smooth-face spherical shaped particles. Although the rough-face spherical shaped particles in

this sample are similar to those in the sample of MMgO/MV2O5=1, however the dominance of these

particles in the sample is lesser as shown in Fig. 4.13-ii(b). Ridge-face spherical shaped particles

are what the as-received 8YSZ powder particles changed to in inhibitive test samples with

MMgO/MV2O5 equal 3 and 5 at 100hrs. This is shown in Fig. 4.13-ii(e-f) and its dominant presence

in the samples is shown in Fig. 4.13-i(c-d). Aside these spherical shaped particles, lump-shaped

structures either attached to the surface of the spherical particles or not; also appear in the SEM

micrographs of the inhibitive test samples with MMgO/MV2O5 equal 2, 3 and 5 respectively.

4.3.4 EDS Analysis

Elemental distribution analysis in the powder particles for MgO-inhibitive hot corrosion powder

samples using EDS are shown in Figures 4.14-4.15. The bar shaped structures in the rough-face

79

Figure 4.9: MgO-inhibitive vanadium-induced hot corrosion 8YSZ powder samples with varying

MgO/V2O5 molar concentration (MMgO/MV2O5) colour spectrum at 900oC isothermal heating. (a)

Heated 8YSZ at 100hrs, (b) Vanadium-induced hot corrosion 8YSZ at 2hrs, (c) Vanadium-

induced hot corrosion 8YSZ at 100hrs, (d) 8YSZ mixture sample of MMgO/MV2O5=1 at 100hrs,

(e) 8YSZ mixture sample of MMgO/MV2O5=2 at 100hrs, (f) 8YSZ mixture sample of

MMgO/MV2O5=3 at 100hrs, (g) 8YSZ mixture sample of MMgO/MV2O5=5 at 100hrs

Table 4.2: Colour scale of MgO-inhibitive vanadium-induced hot corrosion 8YSZ powder

samples with varying MgO/V2O5 molar concentration (MMgO/MV2O5) at 100hrs-900oC isothermal

heating

Powder sample particles at 100hrs L* a* b* dL* da* db*

8YSZ 78.97 -3.17 16.17 78.97 -3.17 16.17

8YSZ + V2O5 (10% wt. 8YSZ) 34.19 22.14 22.19 -44.78 25.31 6.02

8YSZ +MgO (MMgO/MV2O5 =1) +

V2O5 (10% wt. 8YSZ)

57.11 15.09 39.61 -21.87 18.26 23.44

8YSZ+ MgO (MMgO/MV2O5 =2) +

V2O5 (10% wt. 8YSZ)

69.75 6.73 38.30 -9.22 9.91 22.13

8YSZ + MgO (MMgO/MV2O5 =3) +

V2O5 (10% wt. 8YSZ)

73.90 1.40 28.99 -5.07 4.57 12.82


V2O5 (10% wt. 8YSZ)

78.32 -1.70 16.44 -0.65 1.47 0.27

80

(i)

(ii)


sample with MgO/V2O5 molar concentration (MMgO/MV2O5) equals 1 at 900oC isothermal

heating, (ii) XRD analysis of MgO-inhibitive vanadium-induced hot corrosion 8YSZ powder


heating. (a) As-received 8YSZ, (b) 2hrs heating, (c) 5hrs heating, (d) 20hrs heating, (e) 50hrs

heating, (f) 100hrs heating

81

(i)

(ii)

Figure 4.11:(i) XRD analysis of MgO-inhibitive vanadium-induced hot corrosion 8YSZ powder


heating, (i) XRD analysis of MgO-inhibitive vanadium-induced hot corrosion 8YSZ powder


heating. (a) As-received 8YSZ, (b) 2hrs heating, (c) 5hrs heating, (d) 20hrs heating, (e) 50hrs

heating, (f) 100hrs heating

82

Figure 4.12: Amount of tetragonal zirconia phase transformation against time for MgO-inhibitive

vanadium-induced hot corrosion 8YSZ powder samples at 900oC isothermal heating. (8YSZ)

Isothermal heated 8YSZ powder sample, (5MgO(10% V2O5)) 8YSZ sample with

MMgO/MV2O5=5, (3MgO(10% V2O5)) 8YSZ sample with MMgO/MV2O5=3, (2MgO(10% V2O5))

8YSZ sample with MMgO/MV2O5=2, (1MgO(10% V2O5)) 8YSZ sample with MMgO/MV2O5=1,

(10% V2O5) 8YSZ+V2O5 (10% wt. 8YSZ)

83

(i)

(ii)

Figure 4.13: SEM micrographs of MgO-inhibitive vanadium-induced hot corrosion 8YSZ

powder samples with varying MgO/V2O5 molar concentration ratio at 100hrs-900oC isothermal

heating. (a) Heated 8YSZ, (b) Vanadium-induced hot corrosion 8YSZ sample (V2O5=10% wt.

8YSZ), (c) Sample with MMgO/MV2O5 =1, (d) Sample with MMgO/MV2O5=2, (e) Sample with

MMgO/MV2O5=3, (f) Sample with MMgO/MV2O5=5

84

spherical particles as shown in Fig. 4.14 in spectra A and B have large weight composition of

zirconium and oxygen elements, but yttrium that is supposed to be distributed along the same

trend with the zirconium is absent. The bar structures as seen growing out of the spherical shaped

particles is the m-ZrO2 from t-ZrO2 transformation due to the reaction of the yttria stabilizer. The

spectra C and D in the same Fig. 4.14 indicate large weight composition of vanadium, yttrium

and oxygen elements in the irregular shaped structures. These structures can be seen as YVO4

structures formed from the reaction between the yttria stabilizer and V2O5. The lump-like

structures show large weight concentration of magnesium, vanadium and oxygen elements as

seen in spectra E and F in Fig. 4.14 and spectra B and C in Fig. 4.15 respectively. The lump

structures depict magnesium vanadate compounds formed from the reaction between MgO and

V2O5. The ridge-face spherical shaped particles as shown in the powder samples with

MMgO/MV2O5 equal 3 and 5 contain large weight composition of zirconium, yttrium and oxygen

elements as shown in spectrum A in Fig. 4.15. It indicates the presence of the yttrium element

with the zirconium; this portrays these structures as yttria stabilized zirconia phase in line with

the XRD analysis for these samples. It is worthy of note that the suggestion of compounds

formed based on concentration of elements present in the powder particles is in-line with the

XRD analysis were such compound peaks have been identified in the samples.

4.3.5 X-ray mapping analysis

X-ray mapping elemental analysis of the powder particles in MgO-inhibitive hot corrosion

samples is shown in Fig. 4.16. It shows elemental distribution in the smooth-face, ridge-surface,

rough-face spherical shaped and lump shaped powder particles. Like in the EDS analysis, the

lump shaped particles indicate high presence of magnesium, vanadium and oxygen elements as

shown in Fig. 4.16. It validates the earlier statement that these structures are magnesium

85

vanadate compounds formed from the reaction between MgO and V2O5. In Fig. 4.16(b), the

smooth-face and ridge-face spherical shaped particles depict yttria stabilized zirconia phase due

to high evenly distributed concentration of zirconium, yttrium and oxygen elements. The rough-

face spherical shaped particles as shown from previous SEM and EDS analysis; mixture of the

bar like and irregular shape structures indicate evenly dominance of the Zr, Y, V and O elements.

This is shown in Fig. 4.16(a). It depicts the presence of monoclinic zirconia phase with the

YVO4 compound muddle together in the rough-face spherical shaped particles. Like earlier

mentioned for the EDS analysis, suggestion of compounds based on elemental distribution is

based on the XRD analysis and expected products from such powder samples combination.

4.4 MgO-inhibition of Vanadium-induced Hot Corrosion of 8YSZ powder with

lower V2O5 concentration (5% wt. 8YSZ)


Figure 4.17 and Table 4.3 show the colour spectrum analysis of powder particles at 100hrs-

900oC isothermal heating for 8YSZ+V2O5(5% wt. 8YSZ)+MgO samples; with MgO-V2O5 molar

concentration ratios equal 2, 3 and 5 respectively. Figure 4.17(a) represents isothermal heated

8YSZ powder particles showing a whitish colour appearance and the brownish powder particles

in Fig. 4.17(b) are vanadium-induced 8YSZ particles with V2O5 concentration (5% wt. 8YSZ).

The MgO-inhibitive sample of MMgO/MV2O5=5, Fig. 4.17(e) portrays a whitish colour appearance

almost similar to the heated 8YSZ powder. However sample of MMgO/MV2O5=3, Fig. 4.17(d)

shows a yellowish colour particles and that of sample with MMgO/MV2O5=2, Fig. 4.17(c) appears

dark yellowish which is closer in colour to the vanadium-induced 8YSZ sample. Colour scale of

the powder samples is shown in Tab. 4.3.

86

a b

Spectrum O Mg Y V Zr Total

A 26.0 0.0 0.0 1.0 73.0 100.0

B 32.4 0.1 0.0 1.5 66.0 100.0

C 22.3 0.3 28.2 17.2 31.9 100.0

D 16.2 0.0 5.2 3.5 75.1 100.0

E 33.4 29.3 0.0 37.3 0.0 100.0

F 27.7 21.5 0.0 48.1 2.7 100.0

Figure 4.14: EDS analysis of MgO-inhibitive vanadium-induced hot corrosion samples. (a)

Rough-face spherical shaped particles, (b) Lump shaped powder particles

87

Spectrum O Mg Y V Zr Total

A 39.4 1.2 3.1 0.7 55.7 100.0

B 45.3 30.2 0.0 23.0 1.6 100.0

C 37.5 27.2 0.4 24.8 10.1 100.0

Figure 4.15: EDS analysis of Ridge-face spherical shaped powder particles in MgO-inhibitive

vanadium-induced hot corrosion samples

88

(a)

(b)

Figure 4.16: (a) X-ray mapping elemental analysis of MgO-inhibitive vanadium-induced hot

corrosion sample (Smooth-face spherical, rough-face spherical and lump shaped particles), (b)

X-ray mapping elemental analysis of MgO-inhibitive vanadium-induced hot corrosion sample

(Ridge-face spherical shaped and lump shaped particles)

89

Figure 4.17: Physical colour appearance of powder particles for the MgO-inhibitive hot

corrosion samples at 100hrs-900oC isothermal heating. (a) Heated 8YSZ, (b) Vanadium-induced

hot corrosion 8YSZ, (c) 8YSZ powder sample (MMgO/MV2O5 =2), (d) 8YSZ powder sample

(MMgO/MV2O5 =3), (e) 8YSZ powder sample (MMgO/MV2O5 =5)

Table 4.3: Colour scale of MgO-inhibitive vanadium-induced hot corrosion of 8YSZ+V2O5 (5%

wt. 8YSZ)+MgO samples with varying MMgO/MV2O5

Powder particles at 100hrs-900oC

isothermal heating


8YSZ 80.72 -2.11 16.63 80.72 -2.11 16.63

8YSZ + V2O5 (5% wt. 8YSZ) 60.03 6.23 38.79 -19.83 8.96 25.30


V2O5 (5% wt. 8YSZ)

75.16 9.03 41.81 -5.55 11.15 25.18


V2O5 (5% wt. 8YSZ)

79.88 3.81 23.37 -0.84 5.92 16.74

8YSZ+MgO (MMgO/MV2O5 =5)

+V2O5(MMgO/MV2O5 =5) (5% wt.

8YSZ)

79.27 -1.27 19.90 -1.45 0.84 3.27

90

Sample of MMgO/MV2O5=5 shows proximity in lightness, greenness and yellowness to the

isothermal heated 8YSZ which is the reference sample. Although decrease in MMgO/MV2O5 of the

MgO-inhibitive samples reduces the lightness, however the redness and yellowness of the

samples increases. Hence, sample with MMgO/MV2O5=3 is lighter and of lower redness and

yellowness compared to sample with MMgO/MV2O5=2, while the vanadium-induced 8YSZ shows

the least lightness and highest redness and yellowness compared to other samples.

4.4.2 XRD Analysis

MgO-inhibition of vanadium-induced hot corrosion of 8YSZ powder in the presence of lower

V2O5 concentration (5% wt. 8YSZ) portrays the same trend as with that of higher V2O5

concentration (10% wt. 8YSZ). This is shown in XRD analysis in Fig. 18. The XRD spectra for

sample of MMgO/MV2O5=2, shows the same trend with that of sample with the same MMgO/MV2O5

but with different V2O5 concentration (10% wt. 8YSZ). The XRD analysis as shown in Fig.

4.18(i) indicates the growth of YVO4 and new monoclinic zirconia peaks within few minutes of

900oC isothermal heating as shown by the XRD spectra. The XRD spectra also show

consequential increase and decrease in the intensities of the monoclinic zirconia; ( , 1, 1) and

(111) planes and tetragonal zirconia; (101) plane peaks respectively. Mg2V2O7 peaks also appear

in the inhibitive hot corrosion spectra as the expected magnesium vanadate compound. Though

the inhibitive samples have the same features, but the magnitude of these features in sample with

V2O5 concentration (5% wt. 8YSZ), is not as intense compared to that of the sample with V2O5

concentration (10% wt. 8YSZ), Fig. 4.10(b) considering the effect of V2O5 concentration in hot

corrosion of 8YSZ. The XRD spectra of samples with increased MMgO/MV2O5 to 3 and 5 are

shown in Figure 4.18(ii-iii) respectively. The spectra just like for sample with high V2O5

91

concentration and of the same MMgO/MV2O5, Fig. 11 show constant intensities for the tetragonal

and monoclinic zirconia phase peaks for the sample with MMgO/MV2O5=5. Although the sample

with MMgO/MV2O5=3 shows minute traces of YVO4 compound in the inhibitive XRD spectra after

several hours of 900oC isothermal heating, however both XRD analyses show the presence of

Mg3V2O8 peaks in the inhibitive spectra. The amount of tetragonal zirconia transformation plots

for the MgO-inhibitive vanadium-induced hot corrosion samples with lower V2O5 concentration

(5% wt. 8YSZ) using equation 4.1 are shown in Fig. 4.19. The data for the plots are obtained

from the XRD spectra in Figs. 4.18, 4.3(i) and 4.4(i). The plots portray the same trend as that of

sample with high V2O5 concentration; the higher the MMgO/MV2O5 in the sample the lower the

volume fraction of the monoclinic zirconia in the sample and vice versa. Thus sample with

MMgO/MV2O5=5 shows the lowest volume fraction of monoclinic zirconia compared to other

inhibitive samples and is next to that of 900oC isothermal heated 8YSZ plot. The sample with

MMgO/MV2O5=3 shows lower volume fraction of monoclinic zirconia compared to sample with

MMgO/MV2O5=2. The amount of tetragonal zirconia transformation plot for sample with

MMgO/MV2O5=2 is closer to that of the vanadium-induce hot corrosion 8YSZ powder sample with

V2O5 concentration (5% wt. 8YSZ).

4.4.3 SEM Analysis

SEM micrographs of MgO-inhibition of vanadium-induced hot corrosion of 8YSZ powder

samples with lower V2O5 concentration (5% wt. of 8YSZ) at 100hrs-900oC isothermal heating

are shown in Fig. 4.20. Figure 4.20(i) represents the SEM micrographs for sample with

MMgO/MV2O5=2, it depicts spectrum of smooth-face and rough-face spherical shaped particles.

Figures 4.20(ii-iii) are micrographs for samples with MMgO/MV2O5 equal 3 and 5 respectively.

92

The micrographs for both samples show huge presences of ridge-face spherical shaped powder

particles. A comparison of SEM micrographs of the powder particles for MgO-inhibitive

vanadium-induced hot corrosion 8YSZ powder samples for both low and high V2O5

concentrations (10% and 5% wt. 8YSZ) with the same MMgO/MV2O5 are shown in Fig. 4.21. The

micrographs show resemblance in the morphology of the powder particles for samples at low and

high V2O5 concentration with the same the MMgO/MV2O5 i.e. Fig. 4.21(i) shows samples with

MMgO/MV2O5=2 depicting mixture of smooth-face and rough face spherical shaped particles.

Likewise in Figs. 4.21(ii-iii), the micrographs for samples with MMgO/MV2O5 equal 3 and 5

respectively shows similarity for both high and low V2O5 concentration samples. The

micrographs show dominant presence of ridge-face spherical powder particles in all the samples.

4.5 CaO-inhibition of Vanadium-induced Hot Corrosion of 8YSZ powder with

V2O5 concentration (10% wt. 8YSZ)

4.5.1 Colour Spectrum analysis

Figure 4.22 shows the colour appearance of the powder particles at 100hrs-900oC isothermal

heating for CaO-inhibitive vanadium-induced hot corrosion of 8YSZ powder samples. Fig

4.22(a) shows whitish colour powder particles of the isothermal heated 8YSZ powder sample at

100hrs which is the same colour as the as-received 8YSZ powder. Figs. 4.22(b) and (c) represent

vanadium-induced hot corrosion sample at 2hrs and 100hrs respectively; brownish colour

particles. The addition of CaO into the hot corrosion sample of 8YSZ+V2O5 however changes

the colour from brownish to whitish or yellowish colour depending on the CaO/V2O5 molar

concentration (MCaO/MV2O5).

93

(i)

(ii)

(iii)

Figure 4.18: XRD analysis of MgO-inhibitive vanadium-induced hot corrosion of 8YSZ powder

samples with V2O5 concentration (5% wt. 8YSZ) and varying MgO/V2O5 molar concentration at

900oC isothermal heating. (i) Sample with MMgO/MV2O5=2, (ii) Sample with MMgO/MV2O5=3, (iii)

Sample with MMgO/MV2O5=5. (a) As-received 8YSZ, (b) 2hrs heating, (c) 5hrs heating, (d) 20hrs

heating, (e) 50hrs heating, (f)100hrs heating.

94

Figure 4.19: Amount of tetragonal zirconia phase transformation for MgO-inhibitive vanadium-

induced hot corrosion 8YSZ powder samples at 900oC isothermal heating. (8YSZ) 8YSZ

powder, (5MgO(5% V2O5)) 8YSZ sample with MMgO/MV2O5=5, (3MgO(5% V2O5)) 8YSZ

sample with MMgO/MV2O5=3, (2MgO(5% V2O5)) 8YSZ sample with MMgO/MV2O5=2,

(8YSZ+5% V2O5) 8YSZ + V2O5(5% wt. 8YSZ)

95

(i)

(ii)

(iii)

Figure 4.20: SEM micrographs of powder particles at 100hrs-900oC isothermal heating for MgO-

inhibitive hot corrosion samples of 8YSZ+V2O5 (5% wt. 8YSZ)+MgO. (i) Sample with

MMgO/MV2O5=2, (ii) Sample with MMgO/MV2O5=3, (iii) Sample with MMgO/MV2O5=5

96

iii iii


heating for MgO-inhibitive vanadium-induced hot corrosion 8YSZ powder samples with low and

high V2O5 concentration (i) Samples with MMgO/MV2O5=2, (ii) Samples with MMgO/MV2O5=3, (iii)

Samples with MMgO/MV2O5=5. (a) Samples of V2O5 concentration (10% wt. 8YSZ), (b) Samples

of V2O5 concentration (5% wt. 8YSZ)

97

The sample of MCaO/MV2O5=1 at 100hrs shows a dark yellowish colour as shown in Fig 4.22(d).

The intensity of the yellowness of the samples however reduces with increase in MCaO/MV2O5 and

ultimately to whitish colour with MCaO/MV2O5=5 as shown in Fig. 4.22(g). Figs. 4.22(e) and (f)

represent the samples with MCaO/MV2O5 equal 2 and 3 respectively.

The colour scale for the CaO-inhibitive samples shown in Figure 4.22 at 100hrs is shown in

Table 4.4. The colour scale was measured with 100hrs-900oC isothermal heated 8YSZ as the

reference sample. The colour scale shows that the higher the MCaO/MV2O5 in the sample, the

closer the colour scale to that of the reference sample. The colour scale value is similar to that of

MgO-inhibitive samples were also the higher the MMgO/MV2O5 in the sample, the closer the

colour scale to that of isothermal heated 8YSZ reference sample. Thus the lightness, greenness

and yellowness of sample with MCaO/MV2O5=5 is closer in magnitude to that of the isothermal

heated 8YSZ and reduction in MCaO/MV2O5 makes the colour scale of the sample farther from the

reference sample and closer the vanadium-induced hot corrosion sample. The vanadium-induced

hot corrosion 8YSZ sample has the greatest deviation from the reference sample in terms of

blackness, redness and yellowness.

4.5.2 XRD Analysis

XRD analyses of using CaO to inhibit vanadium-induced hot corrosion of 8YSZ powder at

900oC isothermal heating are shown in Figs. 4.23-4.24. XRD spectra of CaO-inhibitive sample

of MCaO/MV2O5=1 are shown in Fig. 4.23(i). The inhibitive spectra unlike the as-received 8YSZ

powder XRD spectrum, Fig. 4.23-i(a) show new peaks indicating monoclinic zirconia, YVO4

and CaV2O6 (PDF card number 00-023-0137) within few minutes of heating as shown in

spectrum for 2hrs. CaV2O6 is the expected calcium vanadate reaction product with MCaO/MV2O5

98

equals 1. The sample spectra hence show consequential increase and decrease in the intensities

of m-ZrO2; (111) and ( , 1, 1) and t-ZrO2; (101) plane peaks respectively. Although the XRD

analysis when compared to that of vanadium-induced 8YSZ sample, Fig. 4.4(ii) depicts reduced

rate of change in the intensities of m-ZrO2; (111) and ( , 1, 1) and t-ZrO2; (101) plane peaks,

however the degradation of the 8YSZ powder is still obvious despite the presence of CaO. The

increase in the presences of CaO in the sample to MCaO/MV2O5=2 further retards the changes in

the intensities of the monoclinic and tetragonal peaks as seen in the XRD spectra, Fig. 4.23(i).

The sample however still shows minute growth of new peaks corresponding to YVO4 and

monoclinic zirconia as shown in Fig. 4.23(ii). The presence of Ca2V2O7 peaks (PDF card number

00-052-1478) as the CaO-V2O5 reaction product rather than CaV2O6 are also shown in the

inhibitive spectra. The inhibitive spectra show resemblance of the as-received 8YSZ spectrum in

terms of the intensity and phase peaks for sample with MCaO/MV2O5=3. The presence of Ca3V2O8

(PDF card number 01-071-3318) is also shown in the spectra and neither the growth of new

peaks indicating YVO4 nor m-ZrO2 appear in the spectra as seen in Fig. 4.24(i). With increase in

MCaO/MV2O5 in the sample to 5 as shown Fig. 4.24(ii), the peaks intensities of monoclinic

zirconia and tetragonal zirconia remain constant throughout the inhibitive hot corrosion tests like

that of sample with MCaO/MV2O5=3. Also like in sample with MCaO/MV2O5=3, neither YVO4 peaks

nor new peaks corresponding to m-ZrO2 phase are present. The presence of Ca3V2O8 peaks are

also shown in the spectra as the expected calcium vanadate compound.

The amount of t-ZrO2 phase transformation in the 8YSZ powder as a result of isothermal

heating, vanadium-induced corrosion and CaO-inhibitive hot corrosion test with 10wt.% V2O5

using equation 4.1 is shown in Figure 4.25. The data for the plots are from the XRD spectra in

99

Figs. 4.23-4.24, 4.3(i) and 4.4(ii). The plots for isothermal exposed 8YSZ sample and the

inhibitive samples with MCaO/MV2O5 equal 3 and 5 respectively have the least values for volume

fraction of m-ZrO2 and show almost equal values for the selected periods in the test. This is in

accordance with the XRD analysis where the inhibitive spectra of the samples show the same

trend with that of the as-received 8YSZ spectrum. The plot for sample with MCaO/MV2O5=2

however shows little volume fraction of m-ZrO2 as compared to the samples with MCaO/MV2O5

equal 3 and 5, but lower than that of sample with MCaO/MV2O5=1 and vanadium-induced sample.

The plot for sample with MCaO/MV2O5=1 indicates the presence of noticeable amount of volume

fraction of the m-zirconia phase as shown in the figure 4.25. The plot for vanadium-induced hot

corrosion 8YSZ sample occurs at the other extreme to the isothermal heated 8YSZ sample plot.

It shows huge transformation of the tetragonal zirconia with the volume fraction of monoclinic

zirconia phase attaining a unit value at 50hrs-900oC isothermal heating.

4.5.3 SEM Analysis

SEM micrographs of isothermal heated 8YSZ, vanadium-induced 8YSZ and CaO-inhibitive

samples particles at 100hrs-900oC isothermal heating are shown in Figure 4.26. The morphology

of isothermal heated 8YSZ sample as shown in previous figures is shown in Figs. 4.26(a); a

smooth-face spherical shaped particles which are resemblance of the as-received 8YSZ particles.

Likewise the powder particles for vanadium-induced 8YSZ sample with V2O5 concentration

(10% wt. 8YSZ) are shown in Figure 4.26(b) for ease comparison with that of CaO-inhibitive

samples. The vanadium-induced 8YSZ particles depict a rough-face spherical morphology

100

Figure 4.22: CaO-inhibitive vanadium-induced hot corrosion 8YSZ powder samples with

varying CaO/V2O5 molar concentration (MCaO/MV2O5) colour appearance at 900oC isothermal

heating. (a) Isothermal heated 8YSZ at 100hrs, (b) Vanadium-induced hot corrosion sample at

2hrs, (c) Vanadium-induced hot corrosion sample at 100hrs, (d) Sample with MCaO/MV2O5=1 at

100hrs, (e) Sample with MCaO/MV2O5=2 at 100hrs, (f) Sample with MCaO/MV2O5=3 at 100hrs, (g)

Sample with MCaO/MV2O5=5 at 100hrs

Table 4.4: Colour scale of powder particles for CaO-inhibitive hot corrosion samples of 8YSZ+

V2O5 (10% wt. 8YSZ)+CaO with varying MCaO/MV2O5

Powder particles at 100hrs L* a* b* dL* da* db*

8YSZ 79.88 -2.66 12.78 79.88 -2.66 12.78

8YSZ + V2O5 42.15 17.73 11.41 -37.72 20.39 -1.37

8YSZ + CaO + V2O5

(MCaO/MV2O5 =1)

69.55 4.37 35.42 -10.33 7.03 22.64

8YSZ + CaO + V2O5

(MCaO/MV2O5 =2)

74.28 -0.17 35.55 -5.59 2.49 22.76

8YSZ + CaO + V2O5

(MCaO/MV2O5 =3)

75.41 -2.65 17.48 -4.47 0.01 4.70

8YSZ + CaO + V2O5

(MCaO/MV2O5 =5)

79.77 -2.07 11.84 -0.11 0.59 -0.95

101

(i)

(ii)


samples at 900oC isothermal heating. (i) Sample of MCaO/MV2O5=1, (ii) Sample of

MCaO/MV2O5=2. (a) As-received 8YSZ, (b) 2hrs heating, (c) 5hrs heating, (d) 20hrs heating, (e)

50hrs heating, (f) 100hrs heating

102

(i)

(ii)


samples at 900oC isothermal heating. (i) Sample of MCaO/MV2O=3, (ii) Sample of MCaO/MV2O5=5.

(a) As-received 8YSZ, (b) 2hrs heating, (c) 5hrs heating, (d) 20hrs heating, (e) 50hrs heating, (f)

100hrs heating

103


vanadium-induced hot corrosion 8YSZ powder samples at 900oC isothermal heating. (8YSZ)

8YSZ powder, (5CaO(10% V2O5)) Sample of MCaO/MV2O5=5, (3CaO(10% V2O5)) Sample of

MCaO/MV2O5=3, (2CaO(10% V2O5)) Sample of MCaO/MV2O5=2, (1CaO(10% V2O5)) Sample of

MCaO/MV2O5=1, (10% V2O5) 8YSZ+ V2O5 (10% wt. 8YSZ)

104

comprising of facet and irregular structures as shown in previous SEM and EDS analyses. The

rough-face spherical shaped particles are also present in CaO-inhibitive sample with MCaO/MV2O5

equal 1 and 2 as shown in Fig. 4.26(c-d). However, the rough-face spherical shaped particles

unlike that of the vanadium-induced sample comprises of bar structures muddled with irregular

structures. The dominance of these irregular spherical particles is more in sample with

MCaO/MV2O5=1 compared to that of MCaO/MV2O5=2, but lesser when compared to the vanadium-

induced hot corrosion sample. The inhibitive samples with MCaO/MV2O5 equal 3 and 5 depict

smooth-face spherical shaped particles just like the as-received and the isothermal heated 8YSZ

powder; however the particles in sample with MCaO/MV2O5=5 is smoother in appearance

compared to that of MCaO/MV2O5=3; these are shown in Figs. 4.26(e-f). Fig. 4.26(d-f) aside

showing the presence of an spherical morphology particles, indicate conspicuously the presences

of lump-shaped structures attached to the surface of the spherical shaped particles or either as a

standing alone structures.

4.5.4 EDS Analysis

EDS analysis of the spherical powder particles found in CaO-inhibitive vanadium-induced hot

corrosion 8YSZ powder samples with varying CaO-V2O5 molar concentration at 100hrs-900oC

isothermal heating are shown in Figures 4.27-4.28. The elemental distribution analysis of the

smooth-face spherical shaped particles is shown in Fig. 4.27. These particles are majorly found

in inhibitive samples with MCaO/MV2O5 equal 3 and 5 respectively; and few in samples with

MCaO/MV2O5 equal 2 and 1 respectively as shown in Fig. 4.26. The spectra B, C, D and E in Fig.

4.27 show dominant presences of Zr, Y and O elements that depict these particles as yttria

stabilized zirconia. This is in accordance with the XRD spectra for these samples where the

105

tetragonal zirconia peaks show higher intensity relative to that of monoclinic zirconia. This

further demonstrates that the inhibitive samples with MCaO/MV2O5 equal 3 and 5 have majorly

yttria stabilized zirconia particles in the sample after the inhibitive test. The lump structures

show significant presence of Ca, V and O elements as shown in spectra A and F in Figs. 4.27. It

portrays these structures as a reaction product between CaO and V2O5. These are the expected

calcium vanadate compounds in the structures due to acid-base reaction. The bar structures

which are common feature in the rough-face spherical shaped particles indicate large presence of

Zr and O elements as shown in spectra A,B and C in Fig. 4.28. This portrays these structures as

the monoclinic zirconia phase due to the absence of Y element that is supposed to be present

with the Zr element. However Y element shows dominant presence with V element in the flake-

like structures muddle with these bar structures as shown in spectra D, E and F in Fig. 4.28. This

depicts the YVO4 compound formed due to reaction of the stabilizer with V2O5. Like earlier

mentioned, suggestion of compound is based on correlating the EDS analysis with the XRD

analysis of the samples

4.5.5 X-ray Mapping

Elemental distribution of Zr, Ca, Y, V and O elements in CaO- inhibitive sample particles at

100hrs using X-ray mapping analysis is shown Fig. 4.29. The rough-face spherical shaped

particles which are present in samples with MCaO/MV2O5 equal 1 and 2, show evenly distribution

of Zr, Y, V and O in Fig. 4.29(i). This is an indication that the rough surface particles comprises

of phases formed from these elements, obviously the monoclinic zirconia and YVO4 as earlier

shown in the XRD and EDS analyses. However unlike the irregular spherical particles, the

smooth-face spherical shaped particles indicate even distribution of Zr, Y and O elements as

106

shown in Fig. 4.29(ii). The presence of Y alongside the Zr element distribution portrays these

smooth-face spherical particles as yttria stabilized zirconia. The lump shaped structures as shown

in Fig. 4.29, shows evenly distribution of Ca ,O and V elements that portrays it as calcium

vanadate compounds.

4.6 CaO-inhibition of Vanadium-induced Hot Corrosion of 8YSZ powder with

lower V2O5 concentration (2% wt. 8YSZ)


Physical colour appearance of powder particles of CaO-inhibitive 8YSZ powder samples with

V2O5 concentration (2% wt. 8YSZ) and varying CaO-V2O5 molar concentration ratio is shown in

Fig. 4.30. The particles depict the samples at 100hrs-900oC isothermal heating. Fig. 4.30(a)

represents the powder particles of isothermal heated 8YSZ, portraying a whitish colour

appearance. The powder particles for the vanadium-induced 8YSZ powder sample with V2O5

concentration (2% wt. 8YSZ) is shown in Fig. 4.30(b). It indicates a change in colour of the

whitish as-received 8YSZ powder particles to brownish colour particles. Fig. 4.30(c), also

brownish in colour like the vanadium-induced 8YSZ powder particles is the sample that contains

mixture of 8YSZ, V2O5 (2% wt. 8YSZ) and CaO (MCaO/MV2O5=1). Figure 4.30(d) depicts

yellowish powder particles for sample with increase in MCaO/MV2O5 to 2 and that of sample of

MCaO/MV2O5=3 is the light yellowish powder particles shown in Fig. 4.30(e). Colour scale for the

samples with isothermal heated 8YSZ as the reference sample is shown in Tab. 4.5. The

measured values like those of samples with high V2O5 concentration (10% wt. 8YSZ), depict

increase in lightness and greenness but decrease in redness with increase in MCaO/MV2O5 in the

107

(i)

(ii)

Figure 4.26: SEM micrographs of powder particles in CaO-inhibitive vanadium-induced hot

corrosion 8YSZ powder samples at 100hrs-900oC isothermal heating. (i) Lower magnification,

(ii) Higher Magnification. (a) Isothermal heated 8YSZ, (b) Vanadium-induced hot corrosion

8YSZ , (c) 8YSZ sample with MCaO/MV2O5=1, (d) 8YSZ sample with MCaO/MV2O5=2, (e) 8YSZ

sample with MCaO/MV2O5=3, (f) 8YSZ sample with MCaO/MV2O5=5

108

8YSZ sample with MCaO/MV2O5=3 8YSZ sample with MCaO/MV2O5=5

Spectrum O Ca Y V Zr Total

A 42.5 27.8 0.1 25.2 4.4 100.0

B 39.6 1.1 3.8 0.9 54.6 100.0

C 16.6 0.9 4.8 0.5 77.3 100.0

D 26.6 4.5 4.3 1.2 64.4 100.0

E 28.9 4.2 4.9 0.5 61.5 100.0

F 67.5 22.7 0.0 2.7 7.1 100.0

Figure 4.27: EDS analysis of smooth-faced spherical shaped particles at 100hrs-900oC

isothermal heating CaO-inhibitive hot corrosion experiment

109

Spectrum O Ca Y V Zr Total

A 20.0 0.7 0.0 2.9 76.4 100.0

B 22.1 0.5 0.0 3.9 73.5 100.0

C 24.4 0.8 0.0 2.4 72.4 100.0

D 29.9 3.1 14.5 13.6 38.9 100.0

E 28.0 1.5 8.6 8.9 53.0 100.0

F 19.3 1.9 7.4 5.6 65.8 100.0

Figure 4.28: EDS analysis of rough-face spherical shaped powder particles at 100hrs-900oC

isothermal heating CaO-inhibitive hot corrosion experiment (8YSZ samples with MCaO/MV2O5

=1 and 2 respectively)

110

(i)

(ii)

Figure 4.29: (i) X-ray mapping analysis of rough-face spherical shaped powder particles at

100hrs-900oC isothermal heating (CaO-inhibitive hot corrosion experiment), (ii) X-ray mapping

analysis of smooth-face spherical shaped powder particles at 100hrs-900oC isothermal heating

(CaO-inhibitive hot corrosion experiment)

111

samples. Hence, sample with MCaO/MV2O=1 shows lower L* and a*, but higher b* values

compared to the sample with MCaO/MV2O5=2 and the same trend comparing samples with

MCaO/MV2O5 equal 2 and 3 respectively. The scale shows that the vanadium-induced sample

showed the least lightness but the highest degree of redness and yellowness.

4.6.2 XRD Analysis

XRD analysis of inhibiting vanadium-induced hot corrosion of 8YSZ powder with lower V2O5

concentration (2% wt. 8YSZ) using CaO as inhibitor is shown in Figures 4.31. Like in samples

with high V2O5 concentration (10% wt. 8YSZ), the inhibitive XRD spectra for sample with

MCaO/MV2O5=1 indicates increase and decrease of monoclinic zirconia (111) and ( , 1, 1) planes

and tetragonal zirconia (101) plane peaks respectively; Fig. 4.31(i). The changes in the intensities

of the peaks are however of minute magnitude due to the little intensity of hot corrosion

associated with the lower V2O5 concentration. Peaks indicating YVO4 and CaV2O6 compound

are also present in the inhibitive spectra as compared to the as-received 8YSZ spectrum. Sample

of MCaO/MV2O5=2 XRD analysis is shown in Fig. 4.31(ii), the inhibitive spectra show little

growth in monoclinic zirconia and decrease in tetragonal zirconia peaks respectively. Minute

growth of YVO4 and Ca2V2O7 peaks are also shown in the inhibitive XRD spectra. The sample

with MCaO/MV2O5=3 indicates a good inhibitive effect with the intensities of peaks of monoclinic

and tetragonal zirconia maintaining constant values through-out the entire isothermal heating

period; Fig. 4.31(iii). The presence of Ca3V2O8 compound as shown in the inhibitive XRD

spectra is the expected calcium vanadate compound. Amount of tetragonal zirconia

transformation plots using equation 4.1 with data obtained from Figs. 4.31, 4.3(i) and 4.4(ii) are

shown in Figure 4.32. The plots portray the same trend like that of samples with higher V2O5

112

concentration, higher transformation rate is shown by samples with lower CaO-V2O5 molar

concentration ratio i.e. sample of MCaO/MV2O5=1. However lower monoclinic zirconia volume

fraction is shown by samples with higher CaO-V2O5 molar concentration ratio i.e. samples with

MCaO/MV2O5 equal 2 and 3 respectively. The vanadium-induced and the isothermal heated 8YSZ

samples are at the two extremes of the plots, with the former showing the highest and the latter

showing the least monoclinic zirconia volume fraction.

4.6.3 SEM Analysis

SEM micrographs of powder particles in CaO-inhibitive 8YSZ powder samples with V2O5

concentration (2% wt. 8YSZ) and varying CaO-V2O5 molar concentration ratio at 100hrs-900oC

isothermal heating are shown Fig. 4.33. It also includes SEM micrographs of isothermal heated

8YSZ and vanadium-induced hot corrosion 8YSZ sample with V2O5 concentration (2% wt.

8YSZ), Fig. 4.33(a) and (b) respectively. The particles of the isothermal heated 8YSZ sample

depict a smooth-face spherical morphology, a resemblance of the as-received 8YSZ powder

particles. The vanadium-induced 8YSZ sample portrays powder particles that show rough-face

spherical morphology and as shown in Fig. 4.6, it comprises of monoclinic zirconia irregular

structures and facet YVO4 structures. The inhibitive sample with MCaO/MV2O5=1, Fig. 4.33(c)

shows powder particles with rough-face spherical morphology, though not as rough and many

when compared to that of sample with high V2O5 concentration but of the same CaO-V2O5 molar

concentration ratio as shown in Fig. 4.34(i). These spherical particles comprise of the bar-shaped

and irregular shaped structures which depict monoclinic zirconia and YVO4 respectively. The

inhibitive sample with MCaO/MV2O5=2 is shown in Fig. 4.33(d) and its comparison with that of

sample with high V2O5 concentration (10% wt. 8YSZ) but of the same CaO-V2O5 molar

113

Figure 4.30: Physical colour appearance of powder particles for the CaO-inhibitive vanadium-

induced hot corrosion 8YSZ powder samples with V2O5 concentration (2% wt. 8YSZ) and

varying MCaO/MV2O5 at 100hrs-900oC isothermal heating. (a) Isothermal heated 8YSZ, (b)

Vanadium-induced 8YSZ, (c) 8YSZ powder sample (MCaO/MV2O5 =1), (d) 8YSZ powder sample

(MCaO/MV2O5 =2), (e) 8YSZ powder sample (MCaO/MV2O5 =3)

Table 4.5: Colour scale of powder particles for the CaO-inhibitive hot corrosion experiments of

mixture of 8YSZ, V2O5 (2% wt. 8YSZ) and CaO


isothermal heating


8YSZ 80.28 -2.05 16.58 80.28 -2.05 16.58

8YSZ + V2O5 (2% 8YSZ) 65.61 4.73 39.78 -14.25 7.47 26.29

8YSZ+ CaO (MCaO/MV2O5 =2) + V2O5

(2% wt. 8YSZ)

54.19 11.51 42.94 -26.09 13.57 20.77

8YSZ+ CaO (MCaO/MV2O5 =2) + V2O5

(2% wt. 8YSZ)

78.42 0.40 30.44 -1.86 2.46 13.85

8YSZ + CaO (MCaO/MV2O5 =3) +

V2O5 (2% wt. 8YSZ)

80.29 -0.10 27.11 0.01 1.96 10.52

114

(i)

(ii)

(iii)


samples with V2O5 concentration (2% wt. 8YSZ) at 900oC isothermal heating. (i) Sample of

MCaO/MV2O5=1, (ii) Sample of MCaO/MV2O5=2, (i) Sampleof MCaO/MV2O5=3. (a) As-received

8YSZ, (b) 2hrs heating, (c) 5hrs heating, (d) 20hrs heating, (e) 50hrs heating, (f) 100hrs heating

115


vanadium-induced hot corrosion of 8YSZ powder samples with V2O5 concentration (2% wt.

8YSZ) at 900oC isothermal heating. (8YSZ) Heated 8YSZ powder, (3CaO(2% V2O5)) Sample

with MCaO/MV2O5=3, (2CaO(2% V2O5)) Sample with MCaO/MV2O5=2, (1CaO(2% V2O5))

Sample with MCaO/MV2O5=1, (8YSZ+2% V2O5) 8YSZ+V2O5 (2% wt. 8YSZ)

116

concentration ratio is shown in Fig. 4.34(ii). The sample particles showed smooth-face spherical

morphology with lump structures attached to its surface, though not as smooth when compared

to the smoothness in particles of as-received 8YSZ powder. The sample of MCaO/MV2O5=3 is

shown in Fig. 4.33(e), it comprises of smooth-face spherical shaped powder particles with lump

shaped particles scattered on its surface. The smooth-face powder particles like in previous EDS

analysis depict tetragonal zirconia phase and the lump shaped structures are the calcium vanadate

compounds; CaV2O6, Ca2V2O7 and Ca3V2O8 formed based on the CaO-V2O5 molar

concentration ratio. The SEM micrograph in Fig. 4.34(iii) is a comparison of samples with

CaO/V2O5 molar concentration equals 3, but with different V2O5 concentration. The two samples

show smooth-face spherical shaped particles at 100hrs-900oC isothermal heating.

4.7 CaO-MgO mixture Inhibition of Vanadium-induced Hot Corrosion of 8YSZ

powder with V2O5 concentration (10% wt. 8YSZ)


Physical colour appearance of powder particles using mixture of MgO and CaO to inhibit

vanadium-induced hot corrosion of 8YSZ powder sample with V2O5 concentration (10% wt.

8YSZ) is shown in Fig. 4.35. Fig. 4.35(d-f) are inhibitive hot corrosion powder samples at

100hrs-900oC isothermal heating compared to that of isothermal heated 8YSZ; Fig. 4.35(a) and

vanadium-induced 8YSZ powder; Fig 4.35(b-c) under the same experimental conditions. Fig.

4.35(d) shows the inhibitive hot corrosion sample with CaO/V2O5 and MgO/V2O5 molar

concentrations equal 1 and 1 respectively. It shows that the powder particles colour after the

inhibitive hot corrosion test is dark yellow. Fig. 4.35(e) shows light yellow powder particles for

powder sample with CaO/V2O5 and MgO/V2O5 molar concentrations equal 1 and 2 respectively

117

and the creamy yellow powder in Fig. 4.35(f); represents the sample with CaO/V2O5 and

MgO/V2O5 molar concentrations equal 2 and 1 respectively. The colour scale analysis of CaO-

MgO mixture inhibitive vanadium-induced hot corrosion 8YSZ powder samples with isothermal

heated 8YSZ as reference sample at 100hrs-900oC isothermal heating is shown in Tab. 4.6. The

sample with CaO/V2O5 and MgO/V2O5 molar concentrations equal 2 and 1 respectively shows

the highest degree of lightness, but least redness and yellowness out of the inhibitive samples. It

is also the closest to the isothermal heated 8YSZ sample in terms of colour scale. The sample

with CaO/V2O5 and MgO/V2O5 molar concentrations equal 1 and 2 respectively colour scale is

next to that of sample with CaO/V2O5 and MgO/V2O5 molar concentrations equal 2 and 1

respectively. The sample with CaO/V2O5 and MgO/V2O5 molar concentrations equal 1 and 1

respectively however shows the least lightness and highest degree redness and yellowness of the

inhibitive samples and is closest to colour scale values of the vanadium-induced sample.

4.7.2 XRD Analysis

XRD analyses of using mixture of CaO and MgO to mitigate 900oC isothermal heating

vanadium-induced hot corrosion of 8YSZ powder with V2O5 concentration (10% wt. 8YSZ) are

shown in Fig 4.36. Fig. 4.36(i) is that of the sample with CaO/V2O5 and MgO/V2O5 molar

concentrations equal 1 and 1 respectively. The inhibitive spectra show the presence of tetragonal

and monoclinic zirconia phase similar to what is in the as-received 8YSZ powder spectrum, Fig.

4.36(i-a); but with some little differences. Unlike the as-received 8YSZ powder spectrum, there

is presence of the monoclinic zirconia phase peaks at some new 2θ value in the inhibitive

spectra. The inhibitive spectra also depict increase and decrease in the intensities of monoclinic

118

Figure 4.33: SEM micrographs of powder particles at 100hrs-900oC isothermal heating CaO-

inhibitive hot corrosion experiments of 8YSZ mixture with V2O5 (2% wt. 8YSZ) and CaO with

varying CaO-V2O5 molar concentration ratio. (a) Isothermal heated 8YSZ, (b) Vanadium-

induced hot corrosion 8YSZ, (c) Sample with MCaO/MV2O5=1, (d) Sample with MCaO/MV2O5=2,

(e) Sample with MCaO/MV2O5=3

119

i

ii

iii


heating for CaO-inhibitive hot corrosion samples with low and high V2O5 concentrations. (i)

Samples with MCaO/MV2O5=1, (ii) Samples with MCaO/MV2O5=2, (iii) Samples with

MCaO/MV2O5=3. (a) V2O5 concentration (10% wt. 8YSZ), (b) V2O5 concentration (2% wt. 8YSZ)

120

zirconia; (111) and ( , 1, 1) plane and tetragonal zirconia; (101) plane peaks respectively. The

inhibitive spectra aside the aforementioned, portray the presence of YVO4 compound and

CaMgV2O7 compound (PDF card number 01-077-0964). The same XRD inhibitive spectra trend

is shown by sample with CaO/V2O5 and MgO/V2O5 molar concentrations equal 1 and 2

respectively, Fig. 4.36(ii) and also for sample with CaO/V2O5 and MgO/V2O5 molar

concentrations equal 2 and 1 respectively, Fig. 4.36(iii). However unlike the inhibitive spectra in

Fig. 4.36(i), the intensities of the YVO4 compound peaks and the rate of increase of the

monoclinic zirconia peaks are minute; the transformation of the tetragonal zirconia phase is

minute.

The plots for amount of tetragonal zirconia transformation using equation 4.1 for CaO-MgO

mixture inhibitive vanadium-induced hot corrosion 8YSZ powder samples are shown in Fig.

4.37. The data used for the plots are obtained from the XRD spectra in Figs. 4.36, 4.3(i) and

4.4(ii). The plots are in line with the XRD analyses, the plot for sample with CaO/V2O5 and

MgO/V2O molar concentrations equal 1 and 1 respectively shows highest volume fraction of

monoclinic zirconia compared to other inhibitive samples. It also shows a better inhibition when

compared to vanadium-induced sample which has the highest volume fraction of monoclinic

zirconia out of all the samples. The sample with CaO/V2O5 and MgO/V2O5 molar concentrations

equal 2 and 1 respectively has lower volume fraction of monoclinic zirconia at 100hrs compared

to that of sample with CaO/V2O5 and MgO/V2O5 molar concentrations equal 1 and 2

respectively. Though the difference is minute, but it further validates that presences of CaO in

the inhibitive sample retards the transformation rate of the tetragonal zirconia when compared to

MgO of the same molar ratio to V2O5.

121

The bar chart showing comparison between the effectiveness of MgO, CaO and mixture of the

two in inhibiting vanadium-induced hot corrosion of 8YSZ powder with V2O5 concentration

(10% wt. 8YSZ) is shown in Fig. 4.38. The bar chart shows measure of the volume fraction of

monoclinic zirconia phase in each of the inhibitive powder sample at 100hrs-900oC isothermal

heating. The chart was obtained using equation 4.1 and data from the XRD spectrum for each

inhibitive sample respectively at 100hrs. The isothermal heated 8YSZ shows the least value for

the volumetric fraction of monoclinic zirconia, while vanadium-induced 8YSZ sample shows the

highest. In all the inhibitive hot corrosion test samples, the sample with CaO-V2O5 molar

concentration ratio equals 5 shows the least monoclinic zirconia volume fraction which is next to

that of the isothermal heated 8YSZ. The highest volume fraction for inhibitive samples is that of

the sample with MgO-V2O5 molar concentration ratio equals 1 and the other inhibitive samples

volume fraction of monoclinic zirconia falls in between these two samples. CaO-inhibitive

samples proved better when compared to MgO-inhibitive samples of the same molar

concentration ratio to vanadium oxide; that is they have lower volume fraction of monoclinic

zirconia when compared to that of MgO-inhibitive samples.

4.7.3 SEM Analysis

SEM micrographs of using CaO-MgO mixture in mitigating vanadium-induced hot corrosion of

8YSZ powder sample with varying CaO/V2O5 and MgO/V2O5 molar concentrations are shown in

Fig. 4.39. The micrographs represent powder particles of the inhibitive samples at 100hrs-900oC

isothermal heating. The smooth-face and rough-face spherical shaped particles as shown in Fig.

4.39(i) depict the particles at 100hrs for inhibitive sample with CaO/V2O5 and MgO/V2O5 molar

concentrations equal 1 and 1 respectively. The rough-face spherical shaped particles comprise of

122

Figure 4.35: CaO-MgO-inhibitive vanadium-induced hot corrosion of 8YSZ powder samples

with varying CaO /V2O5 and MgO/V2O5 molar concentrations colour spectrum at 900oC

isothermal heating. (a) Isothermal heated 8YSZ (100hrs) (b) Vanadium-induced 8YSZ (2hrs), (c)

Vanadium-induced 8YSZ (100hrs), (d) 8YSZ sample (MCaO/MV2O5 : MMgO/MV2O5 =1:1 at

100hrs), (e) 8YSZ sample (MCaO/MV2O5 : MMgO/MV2O5 =1:2 at 100hrs), (f) 8YSZ sample

(MCaO/MV2O5 : MMgO/MV2O5 =2:1 at 100hrs)

Table 4.6:Colour scale of powder particles for CaO-MgO inhibitive vanadium-induced hot

corrosion 8YSZ powder samples


isothermal heating


8YSZ 80.11 -1.98 16.45 80.11 -1.98 16.45

8YSZ + V2O5 (10% wt. 8YSZ) 25.15 19.52 34.84 -54.92 21.50 18.39

8YSZ + CaO + MgO + V2O5

(MCaO/MV2O5 : MMgO/MV2O5 =1:1)

65.16 6.99 40.47 -14.95 8.97 24.02



73.75 1.38 29.00 -6.36 3.36 12.54



71.54 3.60 34.47 -8.57 5.58 18.01

123

(i)

(ii)

(iii)

Figure 4.36: XRD analysis of CaO-MgO-inhibitive vanadium-induced hot corrosion 8YSZ

powder samples with varying CaO/V2O5 and MgO/V2O5 molar concentrations at 900oC

isothermal heating. (i) Sample with MCaO/MV2O5 : MMgO/MV2O5 =1:1, (ii) Sample mixture with

MCaO/MV2O5 : MMgO/MV2O5 =1:2, (iii) Sample mixture with MCaO/MV2O5 : MMgO/MV2O5 =2:1. (a)

As-received 8YSZ, (b) 2hrs heating, (c) 5hrs heating, (d) 20hrs heating,(e) 50hrs heating,

(f)100hrs heating

124

Figure 4.37: Amount of tetragonal zirconia phase transformation against time for CaO-MgO-

inhibitive vanadium-induced hot corrosion 8YSZ powder samples at 900oC isothermal heating.

(8YSZ) 8YSZ powder, (1MgO:2CaO) Sample with MMgO/MV2O5:MCaO/MV2O5=1:2,

(2MgO:1CaO) Sample with MMgO/MV2O5:MCaO/MV2O5=2:1, (1MgO:1CaO) Sample with

MMgO/MV2O5:MCaO/MV2O5=1:1, (10% V2O5) 8YSZ+V2O5 (10% wt. 8YSZ)

125

Figure 4.38: Amount of tetragonal zirconia transformation comparison of 8YSZ samples with

V2O5 conccentration (10% wt. 8YSZ) at 100hrs-900oC isothermal heating in vanadium-induced

hot corrosion inhibitive experiments using different inhibitors

126

bar and irregular shaped structures. 8YSZ sample with CaO/V2O5 and MgO/V2O5 molar

concentrations equal 1 and 2 respectively micrographs are shown in Fig. 4.39(ii). It shows the

sample has large presence of the smooth-face and few rough-face spherical shaped particles. The

sample with CaO/V2O5 and MgO/V2O5 molar concentrations equal 2 and 1 respectively also

shows similar trait of large presence of smooth-face and few rough-face spherical particles as

shown in Fig. 4.39(iii). The presences of lump shaped structures were also seen on the spherical

shape particles as shown in Fig. 4.39.

4.7.4 EDS Analysis

EDS analyses of powder particles of samples in vanadium-induced 8YSZ hot corrosion

inhibitive tests with CaO-MgO mixture at 100hrs-900oC isothermal heating are shown in Figs.

4.40-4.42. Fig. 4.44 represents EDS analysis of rough-face spherical shaped particles for sample

with CaO/V2O5 and MgO/V2O5 molar concentrations equal 1 and 1 respectively. These particles

are few in other CaO-MgO mixture inhibitive samples. Like in the previous mentioned case of

rough-face spherical particles in the inhibitive test samples, that it mainly comprise of the bar-

shaped and irregular shaped structures, same is for these spherical shaped particles in CaO-MgO

inhibitive sample. The bar shaped structure as shown in spectrum C like in the previous analyses

shows zero concentration of yttrium element and large deposition of the zirconium and oxygen

elements; it depicts the monoclinic zirconia phase. In spectrum D, that represent the irregular

structures; there is large presence of yttrium and vanadium elements that portray these structures

as the YVO4 compound. The lump structures as shown in spectra A and B show dominant

presence of oxygen, calcium, magnesium and vanadium which portray it as the calcium-

magnesium vanadate compound; CaMgV2O7 as shown in the XRD spectra. The same elements

127

concentration is also seen in lump structures shown in Figs. 4.41-4.42 for samples with higher

molar concentrations of CaO/V2O5 and MgO/V2O5 combinations. The smooth-face spherical

shaped particles as shown in Fig. 4.42, spectra B and C depict these structures as tetragonal

zirconia phase due to large weight concentration of oxygen, yttrium and zirconia elements.

4.8 Air Plasma Sprayed Specimen Inhibitive Hot Corrosion Test

4.8.1 Physical Appearance

The samples which are sample A; isothermal heated APS coupon, sample B; vanadium-induced

hot corrosion APS coupon and sample C; APS coupon with slurry mixture of MgO-V2O5 powder

with MMgO/MV2O5=3. The other samples are sample D; APS coupon with slurry mixture of MgO-

V2O5 powder with MMgO/MV2O5=5, sample E; APS coupon with slurry mixture of CaO-V2O5

powder with MCaO/MV2O5=3 and sample F; APS coupon with slurry mixture of CaO-V2O5

powder with MCaO/MV2O5=5. Sample B unlike other samples as shown in Fig. 4.47(b) shows total

spallation of the TBC coatings from the substrate, while sample C shows little delamination of

the ceramic topcoat at one of its edges; Fig. 4.43(c) at 100hrs-900oC isothermal heating. The

other samples; A, D, E and F shows neither spallation nor delamination of the TBCs at 100hrs-

900oC isothermal heating as shown in Fig. 4.43(a, d, e and f).

4.8.2 XRD Analysis

Figure 4.44 shows the XRD analyses of the TBCs coupon samples at 100hrs-900oC isothermal

heating inhibitive-hot corrosion test. The XRD spectrum for sample A as shown in Fig. 4.44(a)

shows only peaks for the tetragonal zirconia phase, but sample B aside not showing tetragonal

zirconia peaks indicates the presences of the monoclinic zirconia and YVO4 compounds peaks,

128

(i)

(ii)

(iii)

Figure 4.39: SEM micrographs of CaO-MgO-inhibitive vanadium-induced hot corrosion 8YSZ

powder samples at 100hrs-900oC isothermal heating. (i) Sample with MCaO/MV2O5 : MMgO/MV2O5

=1:1, (ii) Sample with MCaO/MV2O5 : MMgO/MV2O5 =1:2, (iii) Sample with MCaO/MV2O5 :

MMgO/MV2O5 =2:1

129

Spectrum O Mg Ca Y V Zr Total

A 47.5 15.9 13.6 0.0 22.9 0.0 100.0

B 10.2 10.6 28.97 0.0 50.1 0.0 100.0

C 39.0 0.5 0.1 0.0 1.7 58.5 100.0

D 30.5 0.1 2.1 13.5 12.1 41.5 100.0

Figure 4.40: EDS analysis of rough-face spherical shaped powder particles in CaO-MgO-

inhibitive vanadium-induced hot corrosion sample (CaO/V2O5: MgO/V2O5 =1:1)

130


A 35.2 10.0 8.8 0.3 16.5 28.9 100.0

B 44.4 9.4 5.3 4.7 12.7 23.2 100.0

C 44.7 8.7 4.7 2.1 12.3 27.3 100.0


corrosion sample (CaO/V2O5: MgO/V2O5 =1:2)

131


A 47.1 11.1 14.2 0.0 20.2 7.2 100.0

B 16.0 0.4 0.9 4.5 0.5 77.4 100.0

C 18.7 0.0 1.4 7.4 2.9 69.4 100.0

Figure 4. 42: EDS analysis of powder particles in CaO-MgO-inhibitive vanadium-induced hot

corrosion sample (CaO/V2O5: MgO/V2O5 =2:1)

132

Fig.4.44(b). Samples E and F like sample A also show only the presences of tetragonal zirconia

peaks as shown Fig. 4.44i(c-d), the XRD spectrum of sample D however shows minute trace of

peaks corresponding to monoclinic zirconia and YVO4 but huge presences of the tetragonal

zirconia peaks is conspicuously shown. Sample C XRD spectrum as shown in Fig. 4.44(c) shows

the presences of monoclinic zirconia, tetragonal zirconia and YVO4 peaks.

The bar chart showing the amount of tetragonal zirconia phase transformation in each of the

sample using equation 4.1 is shown in Fig. 4.45. The samples A, E and F as shown in the chart

have zero volume fraction of monoclinic zirconia to show no transformation occurred during the

100hrs inhibitive hot corrosion test. Sample D shows negligible monoclinic zirconia volume

fraction as less as 0.09 while that of samples B and C is 1 and 0.37 respectively. The volume

fraction of monoclinic zirconia present in sample B; vanadium-induced sample shows that total

transformation of the tetragonal zirconia occurred during the test within 100hrs which further

validates the earlier results obtained using the powder samples.

Figure 4.46(i) is a comparison of the XRD spectra of solid particles that are present on the TBCs

coupon after the CaO-inhibitive test; samples E and F with that of the as received CaO and V2O5

powder samples. The spectra obviously show that new compound was formed from the slurry

mixture of CaO and V2O5, having seen difference in terms of peaks in these solids particles

spectra compared to that of the as-received CaO and V2O5 XRD spectra. This indicates calcium

vanadate compound (PDF card number 01-071-3318) forms during the 100hrs isothermal

heating time. Fig. 4.46(ii) though for the MgO-inhibitive test samples, samples C and D shows

the same trend; that is the spectra of the solid particles found on these specimens after the test

have different peaks as compared to that of the as-received MgO and V2O5 powder XRD spectra

133

peaks. This confirms that during the cause of the inhibitive isothermal heat test, these powder

particles in the slurry react to form Mg3V2O8 (PDF card number 01-073-0207). It is worthy of

note that the availability of these new compound on the samples after the inhibitive hot corrosion

tests depends on the new compound melting point because the vanadium-induced sample shows

no powder particle on it after the experiment. This is an indication that the V2O5 powder particle

which melting point is 699oC must have melt during the test and diffuse into the ceramic coating

making it to have disappeared after the test.

4.9 Inhibition Efficiency

The inhibition efficiency of the of using alkaline-earth metal oxides; MgO and CaO in inhibiting

vanadium-induced hot corrosion of 8YSZ powder and APS 8YSZ topcoat TBCs coupon has

been evaluated using equation 4.2.

ɛ% = ((A-B)/A)*100 (4.2)

ɛ% = Inhibition efficiency

A = Volume fraction of monoclinic zirconia in vanadium-induced hot corrosion sample

B = Volume fraction of monoclinic zirconia in the inhibitive samples.

Table 4.7 shows the inhibition efficiency for the powder samples. It shows that with increase in

the alkaline-earth metal oxide-vanadium oxide molar concentration ratio, the higher the

inhibition efficiency. Thus good inhibition was achieved in sample with MMgO/MV2O5=5

compared to sample of MMgO/MV2O5=3. Likewise an inhibition efficiency of 92.7% was achieved

in powder sample with MCaO/MV2O5=5 while inhibition efficiency as less as 52.4% was achieved

in sample with MCaO/MV2O5=1 under the same experimental conditions. Fig. 4.47 shows the

134

inhibition efficiency comparison between MgO and CaO in inhibiting vanadium-induced hot

corrosion in powder and APS samples. It shows the superiority of CaO over MgO in inhibiting

vanadium-induced hot corrosion phenomenon. It shows higher inhibition efficiency in CaO-

inhibitive samples compared to the MgO-inhibitive samples of the same molar concentration

ratio to vanadium-oxide. Inhibition efficiency as high as 100% was achieved with CaO inhibitor

while the highest achieved with the use of MgO is 92.6%.

135

Specimen A Specimen B Specimen C

Specimen D Specimen E Specimen F

Figure 4.43: Physical appearance of APS samples after 100hrs-900oC isothermal inhibitive hot

corrosion test

136

(i)

(ii)

Figure 4.44: XRD analysis of APS samples after 100hrs-900oC isothermal heating inhibitive hot

corrosion test. (i) CaO-inhibitive test, (ii) MgO-inhibitive test. (a) Sample A, (b) Sample B, (c)

Sample E, (d) Sample F, (e) Sample C, (f) Sample D

137

Figure 4.45: Amount of tetragonal zirconia phase transformation in the APS TBCs samples at

100hrs-900oC isothermal heating inhibitive hot corrosion test.

138

(i)

(ii)

Figure 4.46: XRD analysis of as-received powders and slurry powders after inhibitive hot

corrosion test. (a) As-received V2O5 powder, (b) As received CaO powder, (c) Slurry powder of

CaO-V2O5 with MCaO/MV2O5=3, (d) Slurry powder of CaO-V2O5 with MCaO/MV2O5=5, (e) As

receieved MgO powder, (f) Slurry powder of MgO-V2O5 with MMgO/MV2O5=3, (g) Slurry powder

of MgO-V2O5 with MMgO/MV2O5=5

139

Table 4.7:Inhibition efficiency of using alkaline-earth metal oxides to inhibits vanadium-induced

hot corrosion of 8YSZ powder

8YSZ Powder Samples with V2O5 concentration (10%

wt. 8YSZ)

Inhibition efficiency (%)

Sample with MMgO/MV2O5=1 1.8




Sample with MCaO/MV2O5=1 41.2




Sample with MCaO/MV2O5: MMgO/MV2O5 equals 1:1 51.9



140

(i)

(ii)

Figure 4.47: Comparison of the inhibitive efficiency of alkaline-earth metal oxides in inhibiting

vanadium-induced hot corrosion of 8YSZ. (i) Vanadium-induced hot corrosion of 8YSZ powder

samples with V2O5 concentration (10% wt. 8YSZ), (ii) APS 8YSZ TBC samples

141

CHAPTER 5

DISCUSSIONS

5.1 The Effect of Isothermal Heating and Cooling

In our study, 2hrs-900oC isothermal heated 8YSZ powder in the absence of V2O5 showed the

same volume fraction of monoclinic zirconia in the as received 8YSZ powder, laboratory air and

furnace cooled samples as shown in Table 5.1. This can be attributed to zirconia phase

temperature dependence phenomenon. The presence of yttria stabilizer in the 8YSZ powder

stabilized the tetragonal phase of the powder; hence at room temperature which is the final

temperature after cooling via furnace and laboratory air showed the same volume fraction of the

zirconia phase with that of the as-received powder. The essence of the yttria stabilizer is to

stabilize the tetragonal 8YSZ zirconia both at higher temperature and room temperature [29].

This result shows that the degradation of the 8YSZ powder is independent of isothermal heating

and cooling, and also effective stabilization of (Y2O3) ZrO2 in the absence of V2O5.

Table 5. 1:Comparison of monoclinic zirconia volume fraction of 2hrs-900oC isothermal heated

8YSZ cooled via laboratory air and furnace

Sample Volume fraction of monoclinic zirconia (%)

As-received 8YSZ powder 3.1

Air cooled 2hrs-900oC Isothermal Heated

8YSZ powder

3.1

Furnace cooled 2hrs-900oC Isothermal Heated

8YSZ powder

3.1

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5.2 Vanadium-induced Hot Corrosion of 8YSZ powder

Isothermal hot corrosion reaction of 8YSZ in the presence of V2O5 at 900oC can be rationalized

based on Lewis acid mechanism [10, 47]. The mechanism proposes the tendency of a strong base

reacting with a strong acid in the presence of other weak acids and bases. V2O5 is a strong acid

that has great affinity to react with the yttria, a strong base in the stabilized zirconia. Although

yttria (Y2O3) and zirconia (ZrO2) are both bases, yttria is of higher basicity [36]; which leads to

its accelerated reaction with V2O5 to form YVO4. Figure 5.1 depicts the level of preferential

reaction of metal oxides with vanadium compounds based on the degree of basicity and acidity

of these compounds. It also suggests the high tendency of Y2O3 compared to ZrO2 in terms of

basicity and the tendency of the V2O5 to preferentially react with the former compared to the

latter. The reaction of the yttria stabilizer with V2O5 causes the transformation of the tetragonal

zirconia to monoclinic zirconia as it is cooled to room temperature. The essence of the yttria

stabilizer is to stabilize the tetragonal 8YSZ zirconia both at higher temperature and room

temperature as shown in Figure 1.3. It is also worth of noting that the reaction of yttria stabilizer

is due to the molten state of the corrosive vanadium salt at the temperature of hot corrosion test.

V2O5 melting point is 690oC [10, 11, 16, 48]. The molten form of the acidic V2O5 at 900

oC

isothermal heating allows it to react with the yttria stabilizer. This explains the reason why in all

the powder samples that have V2O5 concentration (2% wt., 5% wt. and 10% wt. of 8YSZ)

showed YVO4 peaks, increased in monoclinic zirconia intensities peaks, decreased in the

tetragonal zirconia intensities peaks and transformation from the smooth-face to rough-face

powder particles (Figs. 4.3, 4.4 and 4.5). The reaction can be represented as

Y2O3 (ZrO2) (s) +V2O5 (l) 2 YVO4+ZrO2 (m-zirconia) (5.1)

143

The higher the V2O5 concentration the more intense is its reaction with yttria stabilizer. This

made the transformation of tetragonal zirconia to monoclinic zirconia more intense in hot

corrosion test samples with high vanadium oxide concentration. The increase in rate of

transformation of the tetragonal zirconia induced higher volume fraction of the monoclinic

zirconia. Thus higher concentrations of V2O5 contaminant generated higher volume fraction of

monoclinic zirconia phase (Fig. 4.5). This is also shown in Fig. 5.2 where the sample with 2wt.%

V2O5 shows the least volume fraction as compared to those samples that contain 5wt.% and

10wt.% V2O5, respectively. This also explains the reason why we have higher volume fraction of

monoclinic zirconia, higher intensity YVO4 peaks and rough-face spherical powder particles in

sample where the V2O5 is 10% wt. of 8YSZ as compared to that of 5% wt. of 8YSZ. Fig. 5.2 also

shows that the most transformation of the tetragonal zirconia phase occurs within short period of

isothermal heating since major transformation occurred within 2hrs isothermal heating for all the

samples. The figure portrays negligible differences between monoclinic zirconia volume

fractions present after 2hrs and 100hrs isothermal heating in all the samples. The sample with

zero concentration of V2O5 has the least volume fraction of monoclinic zirconia and higher

volume of smooth-face powder particles as compared to other samples (Figs 4.5-4.6).

Figure 5.1: Lewis acid-base preferential reactions of compounds [36]

144

Figure 5.2: Volume fraction of m-ZrO2 produced in vanadium-induced hot corrosion 8YSZ

powder tests as a function of V2O5 concentration and time

5.3 MgO-inhibition of Vanadium-induced Hot Corrosion of 8YSZ powder

The presence of MgO in the reacting medium; (8YSZ+10&5wt.%V2O5) changed the direction of

acid-base reaction based on Lewis acid mechanism. The V2O5 particles reacted with the MgO

particles and formed magnesium vanadate compounds: MgV2O6, Mg2V2O7 and Mg3V2O8

depending on the molar concentration ratio of the two compounds as shown in the phase

diagram, (Fig. 1.11). Magnesium vanadate compounds have different melting points as shown in

the phase diagram, (Fig. 1.11) and reproduced in Table 5.2. The use of MgO as inhibitor in the

8YSZ+10 wt.% V2O5 sample with MMgO/MV2O5=1 did not produce the desired inhibition effect

though it retarded the hot corrosion degradation slightly better compared to MgO-free vanadium-

induced hot corrosion sample. This is clearly shown in the magnitude of monoclinic zirconia

volume fraction in the two samples, (Fig. 4.12). The reason can be attributed to the formation of

MgV2O6 from the reaction between MgO and V2O5, (Figs. 1.11 and 4.10); which is 100% liquid

145

phase at the temperature of the hot corrosion test. The molten nature of MgV2O6 enabled it to

react with the yttria stabilizer to form YVO4 [10, 11, 16, 48].

Table 5.2:Magnesium vanadate compounds and their melting points

Molar

concentration ratio

of MgO:V2O5

Magnesium Vanadate

Compounds

Melting Point Phase Present and

Quantity at 900oC

1:1 MgV2O6 742oC Liquid (100%)

2:1 Mg2V2O7 980oC Liquid (˂10%) and

Solid (>90%)

3:1 Mg3V2O8 1074oC Solid (100%)

Thus the sample indicates high volume fraction of monoclinic zirconia, (Fig. 4.12). This also

brings about considerable surface roughness to the 8YSZ spherical particles, (Figure 4.13). With

increase in molar concentration ratio of MgO-V2O5 in the sample to 2; as well as for sample with

8YSZ+5wt.% V2O5 of MMgO/MV2O5=2, at 900oC a solid-liquid phase of Mg2V2O7 forms, (Figs.

1.11, 4.10 and 4.18). It showed a better inhibition compared to sample with MMgO/MV2O5=1. The

molten phase which was estimated by lever rule to be less than 10% reacted with the yttria

stabilizer and caused the formation of YVO4 and the transformation of the tetragonal zirconia to

monoclinic zirconia phase, (Table 5.2, Fig. 4.10 and Fig. 4.18). The solid phase Mg2V2O7 didn’t

take part in the reaction, thus a good inhibition was achieved compared to when entire liquid

phase MgV2O6 was present. Thus sample of MMgO/MV2O5=2 has lower magnitude of volume

fraction of monoclinic zirconia as compared to that of sample with MMgO/MV2O5=1, (Fig. 4.12).

The sample also showed lower concentration of the roughened spherical powder particles, (Figs.

4.13 and 4.20). The presence of MgO in the samples with MMgO/MV2O5=3 produced a 100% solid

146

phase Mg3V2O8 at 900oC as the acid-base reaction product, (Table 5.2). It gave higher inhibition

as compared to the earlier mentioned two samples with MgO inhibitor in the vanadium-induced

hot corrosion 8YSZ sample, (Figs. 4.12, 4.19 and 4.47). The reason can be linked to the fact that,

the solid Mg3V2O8 didn’t react with yttria stabilizer. Hence the sample showed high

concentration of ridge-face spherical tetragonal zirconia powder particles and low volume

fraction of monoclinic zirconia phase, (Figs 4.12, 4.13, 4.19 and 4.20). However the traces of

YVO4 after like 50hrs isothermal heating and consequential minute transformation of the

zirconia phase, (Fig. 4.11) can be linked to incomplete reaction between the acid and base which

took sometime before it manifested in the inhibitive hot corrosion experiment. With increase in

MMgO/MV2O5 in the sample to 5, it did not only produced 100% solid phase Mg3V2O8 but the

presence of excess MgO, (Figs. 4.11 and 4.20); made the inhibition of vanadium-induce hot

corrosion of the powder sample the most effective, (Table 4.7 and Fig. 4.47). The excess MgO

ensured total reaction with the V2O5 compound eliminating any possibility of V2O5 reacting with

Y2O3. Thus the sample showed neither YVO4 nor zirconia transformation during cooling for the

entire 100hrs isothermal heating inhibitive test as seen in the XRD spectra, (Figs 4.11 and 4.18).

The sample showed the least volume fraction of monoclinic zirconia, almost of the same

magnitude with that of the isothermal heated 8YSZ powder and large presence of the ridge-face

powder particles, (Figs. 4.12, 4.13, 4.19 and 4.20). In summary, the type of magnesium vanadate

compound forming in MgO-inhibitive vanadium-induced hot corrosion 8YSZ sample influenced

the vanadium-induced hot corrosion rate, (Fig. 5.3(i)). Fig. 5.5 shows a schematic flowchart of

the effectiveness of using MgO to inhibit vanadium-induced hot corrosion of 8YSZ powder.

147

5.4 CaO-inhibition of Vanadium-induced Hot Corrosion of 8YSZ powder

CaO similar to MgO is an alkaline-earth metal oxide which is of high basicity compared to Y2O3

and ZrO2. The V2O5 compound reacts with CaO compound to form various calcium vanadate

compounds. The CaO-V2O5 phase diagram shown in Fig. 1.12 indicates the formation of various

calcium vanadate compounds depending on the molar concentration ratios of the reacting

constituents. The phase diagram indicates the formation of CaV2O6, Ca2V2O7 and Ca3V2O8 with

CaO-V2O5 molar concentration ratios (MCaO/MV2O5) of 1, 2 and 3, respectively. Table 5.3 lists

the possible calcium vanadate compounds forming and the melting point of each as well as

fraction of the compound existing in the liquid phase at 900oC estimated by lever rule.

Table 5.3:Calcium vanadate compounds and their meltings

Molar

concentration ratio

of CaO:V2O5

Calcium Vanadate

Compounds

Melting Point Phase Present and

Quantity at 900oC

1:1 CaV2O6 778oC Liquid (100%)

2:1 Ca2V2O7 1015oC Liquid (˂10%) and

Solid (>90%)

3:1 Ca3V2O8 1380oC Solid (100%)

The presence of CaO powder in vanadium-induced hot corrosion 8YSZ-V2O5 samples with

MCaO/MV2O5=1 produced CaV2O6 compound which is 100% in the liquid phase at 900oC. The

molten state of CaV2O6 at 900oC isothermal heating temperature reacted with the yttria stabilizer

(Y2O3) to form YVO4 during the test, (Figs 4.23 and 4.31). This led to destabilization of

tetragonal zirconia to monoclinic zirconia during cooling of the 8YSZ sample to room

temperature as shown in the XRD spectra, (Figs. 4.23 and 4.31). Thus this sample showed

148

appreciable volume fraction of monoclinic zirconia and rough surface morphology spherical

powder particles, (Figs. 4.25, 4.26, 4.32 and 4.33). Although effective inhibition was not

achieved in this sample, the presence of CaO retarded the hot corrosion slightly better compared

to the un-inhibited vanadium-induced hot corrosion sample, (Figs. 4.25, 4.33 and 4.47). The

sample with MCaO/MV2O5=2 produced Ca2V2O7; a solid-liquid mixture phase at 900oC as shown

in Table 5.3, (Figs. 1.12, 4.23 and 4.31). The formation of Ca2V2O7 produced a better inhibitive

effect compared to that of sample with MCaO/MV2O5=1 based on comparing the volume fractions

of monoclinic zirconia of the samples, (Figs. 4.25 and 4.32) and the inhibition efficiency, (Fig.

4.47). The molten phase fraction of Ca2V2O7 at 900oC estimated to be less than 10% reacted with

the yttria stabilizer and formed YVO4 and consequently caused the transformation of the

tetragonal zirconia to monoclinic zirconia phase, (Figs. 4.23 and 4.31). The solid phase Ca2V2O7

didn’t take part in the reaction, thus giving the sample better inhibition compared to sample

where entire liquid phase CaV2O6 was formed. This led to lower concentration of rough surface

spherical particles when compared to that of vanadium-induced and MCaO/MV2O5=1 samples,

(Figs. 4.26 and 4.33). The sample with MCaO/MV2O5=3 is expected to form Ca3V2O8 from acid-

base reaction. Ca3V2O8 is 100% in solid phase at 900oC as inferred from the phase diagram, (Fig

1.12). The elimination of molten salt gave rise to effective inhibition, (Fig. 4.47) accompanied

with lower volume fraction of monoclinic zirconia, (Figs 4.25 and 4.32) and the smooth surfaces

of powder particles similar to the as-received 8YSZ powder particles, (Figs 4.26 and 4.33).

Increasing the amount of added inhibitor to MCaO/MV2O5=5, produced 100% solid phase of

Ca3V2O8 and resulted in very effective inhibition, (Table. 4.7 and Fig. 4.47). The excess CaO

also ensured total reaction with the V2O5 compound and hence least volume fraction of

monoclinic zirconia and high concentration of smooth particles, (Figs 4.25 and 4.26). Similar to

149

MgO, the type of calcium vanadate compound forming depending on the CaO/V2O5 molar

concentration in a CaO-inhibitive vanadium-induced hot corrosion 8YSZ sample influenced the

hot corrosion rate, (Fig. 5.3(ii)). Fig. 5.5 also shows a schematic flowchart of the effectiveness of

using CaO to mitigate vanadium-induced hot corrosion of 8YSZ powder.

5.5 CaO-MgO-inhibition of Vanadium-induced Hot Corrosion of 8YSZ powder

Mixture of CaO and MgO was prepared as a mixed inhibitor for vanadium-induced hot corrosion

of 8YSZ powder. The mixed inhibitor appears to react with V2O5 and formed CaMgV2O7, a

vanadate compound with melting point of 788oC [PDF card number 01-077-0964]. The mixed

inhibitor sample with MCaO/MV2O5=1 and MMgO/MV2O5=1 formed CaMgV2O7 according to

reaction 5.2 and Fig. 4.36. CaMgV2O7 is expected to be 100% molten phase at 900oC. The

molten CaMgV2O7 reacted with yttria stabilizer and formed YVO4, (Fig. 4.36). This led to

appreciable volume fraction of monoclinic zirconia and rough surface particles in the sample,

(Figs. 4.37 and 4.39). It is worth noting that CaMgV2O7 caused a better inhibition compared to

CaV2O6 and MgV2O6, (Fig. 4.38). Firstly, CaMgV2O7 has the highest melting point out of the

three compounds which determines the effectiveness of a compound in inhibiting vanadium-

induced hot corrosion. The higher the melting point of the vanadate compound the better is the

inhibition effect. Although the mixed inhibitor produced a better inhibition when compared to

the use of MgO of the same molar concentration to the vanadium oxide (MMgO/MV2O5=2), the

inhibition efficiency is however lower to that of using CaO alone of the same molar

concentration to V2O5 as inhibitor, (Fig. 4.38). The sample with MCaO/MV2O5=1 and

MMgO/MV2O5=2 aside forming the CaMgV2O7 compound; has excess MgO that further retarded

the hot corrosion effect as shown in reaction 5.3. This sample produced better inhibition

150

compared to the aforementioned CaO-MgO mixed inhibitive sample based on the presence of the

excess MgO, though the molten CaMgV2O7 still induced the hot corrosion products of YVO4

and zirconia transformation, (Fig. 4.36). This is shown in the volume fraction of monoclinic

zirconia and volume of rough surface particles in this sample, (Figs. 4.37 and 4.41). The same

trend occurred for sample of MCaO/MV2O5=2 and MMgO/MV2O5=1, CaMgV2O7 and CaO formed as

shown in reaction 5.4 and Fig. 4.36. The excess CaO further retarded the rate of hot corrosion of

the sample. However at 100hrs, this sample showed lower volume fraction of monoclinic

zirconia compared to the mixed inhibitive sample with excess MgO, (Figs. 4.37 and 4.38). This

can be attributed to the high reactivity of CaO with V2O5 and its ability to form high melting

point vanadate compounds when compared to MgO of the same molar concentration ratio with

V2O5. CaO is of high basicity compared to MgO inferring from the positions of calcium and

magnesium elements in the periodic table and their pKa values, [49]. This is also the reason why

the CaO-inhibitive samples showed better inhibition compared to the MgO-inhibitive samples of

the same molar concentration to V2O5, (Fig. 4.47).

CaO + MgO + V2O5 CaMgV2O7 (5.2)

CaO + 2MgO + V2O5 CaMgV2O7 + MgO (5.3)

2CaO + MgO + V2O5 CaMgV2O7 + CaO (5.4)

The mixed-inhibitive samples produced a better inhibition compared to MgO-inhibitive samples

of the same molar ratio to vanadium oxide but lesser when compared to that of CaO-inhibitive

samples. Fig. 5.4 shows XRD comparison of the mixed-inhibitive samples.

151

5.6 APS Specimen

Sample A like the isothermal heated 8YSZ powder showed neither formation of YVO4 nor

tetragonal zirconia transformation due to absence of the molten acidic V2O5 to elicit such hot

corrosion products. Thus after 100hrs of isothermal heating, it showed zero magnitude

monoclinic zirconia volume fraction and absence of monoclinic zirconia peaks in the XRD

spectrum, (Fig. 4.44 and 4.45). Sample B; the vanadium-induced hot corrosion sample exihibited

tetragonal zirconia transformation due to reaction of the yttria stabilizer in the topcoat with the

molten V2O5 based on Lewis acid mechanism as shown in reaction 5.1. The transformation of

tetragonal zirconia to monoclinic zirconia during cooling caused the spallation of the topcoat,

(Fig. 4.44). The transformation of tetragonal zirconia to monoclinic zirconia is accompanied with

3-5% increase in volume of the zirconia phase [8]; hence the stress which arose initiated the

degradation and spallation of the topcoat. Sample C which is the specimen that has the slurry

mixture of MgO and V2O5 (MMgO/MV2O5=3), though showed the formation of 100% solid phase

Mg3V2O8, (Fig. 4.46); but the presence of YVO4 and approximately 40% monoclinic volume

fraction could have been due to incomplete reaction of MgO with V2O5, (Fig. 4.44). This led to

little degradation at the edges of the topcoat, (Figs. 4.43 and 4.45). Sample D though showed

minute intensity YVO4 peaks and minute volume fraction of monoclinic zirconia but no obvious

degradation like that of sample C, (Figs 4.43-4.45). The same reasons applies, 100% solid phase

Mg3V2O8 was formed from the slurry of V2O5 and MgO during the inhibitive test but the

presence of minute V2O5 due to incomplete reaction induced those hot corrosion features, (Fig.

4.46). The presence of more MgO in slurry of sample D compared to that of specimen C gave it

better inhibition in terms of reduced amount of YVO4 and zirconia transformation, (Fig. 4.44).

Thus it showed lower volume fraction of monoclinic zirconia out of the two samples, (Fig. 4.45).

152

Samples E and F like sample A showed the same trend; neither YVO4 growth nor zirconia

transformation due to the absence of molten reactive salt, (Figs. 4.44 and 4.46). The slurries

mixture of CaO and V2O5 (MCaO/MV2O5=3 and 5, respectively) on the specimens reacted together

and formed 100% solid phase Ca3V2O8 as shown in the XRD spectra of the solid slurries after

the test, (Fig. 4.46). The solid calcium vanadate compound caused neither the growth of YVO4

nor consequential tetragonal zirconia transformation due to its 100% solid phase at 900oC, (Table

5.3 and Fig. 1.12). Thus these samples; sample E and F also showed zero volume fraction of the

monoclinic zirconia phase, (Fig. 4.45). The high inhibition efficiency displayed by the CaO-

inhibitive samples compared to the MgO-inhibitive samples, (4.47) can also be linked to the

higher reactivity of CaO with V2O5 compared to MgO and formation of higher melting point

calcium vanadate compound. Table 5.4 gives a summary of hot corrosion and inhibitive hot

corrosion of the coated samples.

153

Table 5.4:Physical appearance of 8YSZ-APS of the coated samples at 100hrs

Samples Slurries on the coated samples Physical Appearance at

100hrs-900oC isothermal

heating

Sample A N/A Stable 8YSZ-APS

Sample B V2O5 Spallation of 8YSZ-APS

Sample C MgO and V2O5

(MMgO/MV2O5=3)

Little degradation of the

8YSZ-APS

Sample D MgO and V2O5

(MMgO/MV2O5=5)

Stable 8YSZ-APS

Sample E CaO and V2O5

(MCaO/MV2O5=3)

Stable 8YSZ-APS

Sample F CaO and V2O5

(MCaO/MV2O5=5)

Stable 8YSZ-APS

154

(i)

(ii)

Figure 5.3: (i) MgO-inhibitive experiment , (ii) CaO-inhbitive experiment. (a) As-received 8YSZ

powder, (b) Isothermal heated 8YSZ powder, (c) Vanadium-induced hot corrosion sample, (d)

Sample of M(alkaline earth metal oxide/vanadium oxide)=1, (e) Sample of M(alkaline earth metal oxide/vanadium oxide)=2,

(f) Sample of M(alkaline earth metal oxide/vanadium oxide)=3, (g) Sample of M(alkaline earth metal oxide/vanadium

oxide)=5.

155

Figure 5.4: CaO-MgO inhibitive experiment (a) As-received 8YSZ, (b) Isothermal heated 8YSZ,

(c) Vanadium-induced hot corrosion 8YSZ, (d) Sample with MCaO/MV2O5 : MMgO/MV2O5 =1:1, (e)

Sample with MCaO/MV2O5 : MMgO/MV2O5 =1:2, (f) Sample with MCaO/MV2O5 : MMgO/MV2O5 =2:1

156

Figure 5.5: Flow chart on the use of alkaline-earth metal oxides to inhibit vanadium-induced hot

corrosion of 8YSZ

157

CHAPTER 6

CONCLUSIONS

Based on the experimental results using 8wt.% Y2O3-stabilized ZrO2 (8YSZ) powder and air

plasma coated 8YSZ (8YSZ-APS) samples, the following conclusions can be drawn:

1. The addition of vanadium oxide in the amount between 2-10wt.% of 8YSZ powder caused hot

corrosion degradation of the 8YSZ powder with the formation of YVO4 and transformation of

tetragonal zirconia to monoclinic zirconia phase.

2. The higher the concentration of the vanadium oxide the higher was the extent of degradation

in the 8YSZ powder samples.

3. The extent of molten vanadate-induced hot corrosion of the 8YSZ powder was a rapid process

and reached full damage within 1-2 hours and further degradation was sluggish.

4. The addition of MgO to 8YSZ+10wt.% V2O5 and 8YSZ+5wt.% V2O5 mixture powder

samples, respectively inhibited vanadium-induced hot corrosion. Higher molar concentration

ratio of MgO/V2O5 generated higher effective inhibition of vanadium-induced hot corrosion of

8YSZ powders. MgO/V2O5 molar concentration ratio of 5 showed effective inhibition.

5. The incorporation of CaO to 8YSZ+10wt.% V2O5 and 8YSZ+2wt.% V2O5 mixture powder

samples, respectively inhibited vanadium-induced hot corrosion. Higher molar concentration

ratio of CaO/V2O5 generated higher inhibition of vanadium-induced hot corrosion of 8YSZ

powders. CaO/V2O5 molar concentration ratios of 3 and 5 showed effective inhibition.

158

6. Spraying 8YSZ-APS coated samples with slurry of V2O5 solution in the concentration of

20mg/cm2 caused degradation of the 8YSZ-APS with formation of YVO4, transformation of the

tetragonal zirconia to monoclinic zirconia phase and full spallation of the 8YSZ-APS after 100 h

isothermal heating at 900oC.

7. Slurry mixture of MgO+V2O5 with MgO/V2O5 molar concentration ratios of 3 and 5 sprayed

on 8YSZ-APS coated samples in the concentration range of 70-100 mg/cm2, mitigated

vanadium-induced hot corrosion of 8YSZ-APS as compared to the un-inhibited samples after

100 h of isothermal heating at 900oC.

8. Slurry mixture of CaO+V2O5 with CaO/V2O5 molar concentration ratios of 3 and 5 sprayed on

8YSZ-APS coated samples in the concentration range of 70-100 mg/cm2, inhibited vanadium-

induced hot corrosion of 8YSZ-APS as compared to the un-inhibited samples after 100 h of

isothermal heating at 900oC.

9. The use of CaO showed better inhibition effect when compared to MgO of the same molar

concentration ratio to V2O5 in inhibiting vanadium-induced hot corrosion of 8YSZ powder and

8YSZ-APS.

10. Vanadium-induced hot corrosion of 8YSZ powder and 8YSZ-APS; and its inhibition with

MgO and CaO at 900oC isothermal heating is strongly dependent on the fraction of molten salt

present and the acidity of the molten salt (Lewis acid mechanism).

159

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49 Encyclopedia, http://en.wikipedia.org/wiki/

165

VITAE

Name: Gbadamosi, Aliyu Arisekola

Date of Birth: 30th

March, 1980

Nationality: Nigeria

Permanenet address: 26, Niyi Oloyede Str., Alakia-Isebo, Ibadan, Oyo-State, Nigeria

Telephone: +2348065378979, +966532358096

Email Address: [email protected]

Educational Qualification: 1. BSc. Mechanical Engineering (2006),

Obafemi Awolowo Univeristy, Ile-Ife, Nigeria.

2. National Diploma-Mechanical Engineering (2000)

The Polytechnic Ibadan, Oyo-State, Nigeria

Workshops: 1. 10th

Workshop on Clean Water and Clean Energy. June, 2013 (Member of KFUM

Research Group working with MIT Group on Hot Corrosion in Gas Turbine

Engines)

2. 9th

Workshop on Clean Water and Clean Energy. January, 2013 (Member of

KFUM Research Group working with MIT Group on Hot Corrosion in Gas

Turbine Engines)

mailto:[email protected]

dedicated - eprints · iii acknowledgements all praise to almighty allah for his mercy upon us and...

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